Phosphor particle coating

ABSTRACT

The invention provides a method for providing a luminescent particle (100) with a hybrid coating, the method comprising: (i) providing a luminescent core (102) comprising a primer layer (105) on the luminescent core (102); (ii) providing a main ALD coating layer (120) onto the primer layer (105) by application of a main atomic layer deposition process, the main ALD coating layer (120) comprising a multilayer (1120) with two or more layers (1121) having different chemical compositions, and wherein in the main atomic layer deposition process a metal oxide precursor is selected from a group of metal oxide precursors comprising Al, Zn, Hf, Ta, Zr, Ti, Sn, Nb, Y, Ga, and V; (iii) providing a main sol-gel coating layer (130) onto the main ALD-coating layer (120) by application of a main sol-gel coating process, the main sol-gel coating layer (130) having a chemical composition different from one or more of the layers (1121) of the multilayer (1120).

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority of U.S. application Ser. No. 16/915,422titled “Phosphor Particle Coating” filed 29 Jun. 2020 and, EPapplication no. 20189538.0 filed 5 Aug. 2020 titled “Phosphor ParticleCoating,” said applications being incorporated herein by reference intheir entireties.

FIELD OF THE INVENTION

The invention relates to a method for providing a coated luminescentmaterial, to such luminescent material, as well as to a lighting devicecomprising such luminescent material for wavelength conversion.

BACKGROUND OF THE INVENTION

The coating of luminescent materials is known in the art. WO2014128676,for instance, describes a coated luminescent particle, a luminescentconverter element, a light source, a luminaire and a method ofmanufacturing coating luminescent particles. The coated luminescentparticle comprises a luminescent particle, a first coating layer and asecond coating layer. The luminescent particle comprises luminescentmaterial for absorbing light in a first spectral range and forconverting the absorbed light towards light of a second spectral range.The luminescent material is sensitive for water. The first coating layerforms a first barrier for water and comprises a metal oxide or anitride, phosphide, sulfide based coating. The second coating layerforms a second barrier for water and comprises a silicon based polymeror comprises a continuous layer of one of the materials AlPO₄, SiO₂,Al₂O₃, and LaPO₄. The first coating layer and the second coating layerare light transmitting. The first coating layer encapsulates theluminescent particle and the second coating layer encapsulates theluminescent particle with the first coating layer.

SUMMARY OF THE INVENTION

Moisture sensitive luminescent powder materials can be coated with alayer of an amorphous or glassy material to reduce decomposition ratesby moisture attack. The coating may be applied by depositing a materialat the particle surfaces by reacting a dissolved inorganic precursor ina suspension (e.g. by a sol-gel process) or by deposition from the gasphase (e.g. a chemical vapor deposition or an atomic layer deposition(ALD) process).

Atomic layer deposition could be a suitable method to deposit thin,conformal coatings of various inorganic materials on powder particles.ALD layers may be very dense and conformal and may be substantiallyimpermeable to gases like water vapor and oxygen.

The ALD process further allows the deposition of multiple thin layers(nanolaminate) of different inorganic materials that each may providephysical properties to the layer (like moisture resistance, lighttransmissivity, stress resistance, elasticity, etc.) that may bedifferent for the different (nano)layers.

Sol-gel process may be suitable for providing (relatively) thickerlayers that may provide mechanical protection to the material coatedwith the layer.

Known coated luminescent particles may show one or more disadvantages,such as decomposition of the luminescent material due to moisture ore.g. solvents, degradation as a result of high temperatures, mechanicalinstability during processing the luminescent particles. Further, alsomany of the known coating processes have one or more disadvantages suchas agglomeration, decrease in quantum efficiency of the coatedluminescent material (relative to the uncoated material), non-conformalcoatings.

It appears that with a sol-gel coating process only the properties ofthe luminescent materials may not be sufficient. Furthermore, moisturesensitive luminescent particles comprising only an ALD coating may alsonot be durable when being exposed to mechanical stress. It furtherappears that moisture sensitive luminescent particles with a sol-gelcoating in combination with an ALD coating configured on top of thesol-gel coating may provide luminescent particles with a reduceddecomposition rate due to moisture attack at relatively hard conditionssuch as at temperatures up to 60° C. and a 90% relative humidity. Yet,for higher temperatures, such as that may be generated in high-power LEDapplications, e.g. in flash and automotive applications alternativecoating structures may be desirable. Further, ALD layers appear to showan intrinsic (tensile) stress, that may increase with increasing layerthickness. ALD coatings may preferably be provided as thin layers. Yet,it appears that deposition of especially a thin ALD layer may besensitive to surface contamination. Possible contamination at thesurface of the particle to be coated may result in pinholes or otherirregularities in the (thin) ALD layer.

Hence, it is an aspect of the invention to provide an alternativecoating process, which preferably further at least partly obviates oneor more of above-described drawbacks. It is a further aspect of theinvention to provide an alternative luminescent material that preferablyfurther at least partly obviates one or more of above-describeddrawbacks. In yet a further aspect, the invention provides a lightingdevice comprising the luminescent material that preferably further atleast partly obviates one or more of above-described drawbacks.

The present invention may have as object to overcome or ameliorate atleast one of the disadvantages of the prior art, or to provide a usefulalternative.

Amongst others, the invention proposes in embodiments a coatingstructure comprising at least two layers, especially at least threelayers configured around a luminescent core. The different layers may beselected from having different functions. The coating structure mayespecially comprise a primer layer, a coating layer provided by atomiclayer deposition (an “ALD coating layer”), and a coating layer providedby a sol-gel (deposition) process (a “sol-gel coating layer”). Theprimer layer may facilitate good adherence between the surface andfacilitate deposition of a thin ALD coating layer. The ALD coating layermay shield the luminescent core from undesired gases like water vaporand oxygen or further chemicals. The sol-gel coating may providemechanical protection to the luminescent core and the ALD coating layer.

Hence, herein a hybrid coating method is provided for a luminescentpowder material that consist of depositing a coating layer at a primerlayer (at a surface of a luminescent core) by application of an ALDprocess and successively depositing a sol-gel layer by application of asol-gel type process to obtain a uniformly coated luminescent particle.With the method a luminescent particle with a hybrid coating may beprovided.

Hence in a first aspect, the invention provides a method for providing aluminescent particle with a hybrid coating. In embodiments, the methodespecially comprises (the stages of) (i) providing a luminescent core(“core”) comprising a primer layer (“primer coating” or “primer coatinglayer”) on the luminescent core (or a “primer layer comprisingluminescent core”). The method further comprising: (ii) providing a(n)(main) atomic layer deposition coating layer (“(main) ALD coating layer”or “(main) ALD coating” or “(main) ALD layer”) onto the primer layer.The (main) ALD-coating layer is in embodiments, especially provided ontothe primer layer comprising luminescent core. The method furthercomprising: (iii) providing a (main) sol-gel coating layer (“(main)sol-gel coating” or “(main) sol-gel layer”) onto the (main) ALD coatinglayer. Further, the (main) ALD coating layer may be provided onto theprimer layer by application of an (main) atomic layer deposition process(“(main) ALD process”). Further, in specific embodiments, the (main) ALDcoating layer may comprise a multilayer (or “laminate”) with two or morelayers having different chemical compositions. In further specificembodiments, in the (main) atomic layer deposition process, a metaloxide precursor is selected from a group of metal oxide precursorscomprising Al, Zn, Hf, Ta, Zr, Ti, Sn, Nb, Y, Ga, and V (and optionallySi). The (main) sol-gel coating layer is especially provided onto the(main) ALD coating layer by application of a (main) sol-gel coatingprocess. In further embodiments, the main sol-gel coating layer may havea chemical composition different from one or more of the layers of themultilayer.

In yet a further aspect, the invention also provides a luminescentmaterial comprising the luminescent particles obtained by such method.Especially, the invention provides in yet a further aspect, aluminescent material comprising luminescent particles, wherein theluminescent particles comprise a luminescent core comprising a primerlayer on the luminescent core, especially wherein the primer layer has aprimer layer thickness (d1) in the range of 0.1-10 nm, especially 0.1-7nm, such as 0.1-5 nm or 0.1-4 nm, and wherein the primer layer has achemical composition differing from the chemical composition of thecore; a(n) (main) ALD (i.e., atomic layer deposition) coating layer,especially comprising a multilayer with two or more layers havingdifferent chemical compositions, wherein, in embodiments, the (main) ALDcoating has a(n) (main) ALD coating layer thickness (d2) in the range of5-250 nm, such as 5-100 nm, especially 5-50 nm, such as especially 10-50nm, even more especially 20-50 nm, and especially wherein the multilayercomprises one or more layers comprising an oxide of one or more of Al,Zn, Hf, Ta, Zr, Ti, Sn, Nb, Y, Ga, and V (and optionally Si), whereinone or more of the two or more layers of the multilayer have chemicalcompositions differing from the chemical composition of the primerlayer, and further, in embodiments, especially comprising a (main)sol-gel coating layer, wherein, in embodiments, the (main) sol-gelcoating has a (main) sol-gel coating layer thickness (d3) in the rangeof 50-700 nm, such as 50-600 nm, especially 75-500 nm, such asespecially 100-500 nm. In further embodiments, the (main) sol-gelcoating layer has a chemical composition differing from the (main) ALDcoating layer, especially from one or more of the two or more layers ofthe multilayer. Further, especially, the (main) ALD coating layer isarranged between the primer layer and the (main) sol-gel layer.

The invention may provide luminescent particles and luminescentmaterial, i.e. luminescent material comprising these (hybrid coated)particles, showing a significantly reduced decomposition rate as aresult of moisture attack. The coating of the luminescent particles maydemonstrate improved moisture barrier properties. The coating mayfurther provide an improved chemical and mechanical stability allowingthe integration of luminescent particle (phosphors), especially ofmoisture sensitive luminescent particles (phosphors) in high-powerproducts e.g. for flash and automotive applications imposing high stressconditions (like working temperatures up to 85° C., at a high relativehumidity (over 80% relative humidity). With such luminescent material, arelative stable luminescent material is provided with quantumefficiencies close to or identical to the virgin (non-coated)luminescent material and having stabilities against water and/or (humid)air which are very high and superior to non-coated or non-hybrid coatedluminescent particles.

The invention may especially provide in embodiments a method forproviding a luminescent particle with a hybrid coating, the methodcomprising: (i) providing a luminescent core comprising a primer layeron the luminescent core; (ii) providing a main ALD coating layer ontothe primer layer by application of a main atomic layer depositionprocess, the main ALD coating layer comprising a multilayer with two ormore layers having different chemical compositions, and wherein in themain atomic layer deposition process a metal oxide precursor is selectedfrom a group of metal oxide precursors comprising Al, Zn, Hf, Ta, Zr,Ti, Sn, Nb, Y, Ga, and V (and optionally Si); (iii) providing a mainsol-gel coating layer onto the main ALD-coating layer by application ofa main sol-gel coating process, the main sol-gel coating layer having achemical composition different from one or more of the layers of themultilayer. Especially, in the main atomic layer deposition process, themetal oxide precursor is selected from a group of metal oxide precursorsof metals selected from a group consisting of Al, Zn, Hf, Ta, Zr, Ti,Sn, Nb, Y, Ga, and V (and optionally Si).

Hence, the starting material is a particulate luminescent material or aluminescent material that is made particulate. Further, especially theluminescent core is a particulate core or luminescent (core) materialthat is made particulate. The core may essentially be a (virgin)luminescent particle/core, i.e. a non-coated/non-treated luminescentparticle. The luminescent particles of the particulate luminescentmaterial (especially the core(s)) are coated as described herein. Theterms “luminescent particles”, “luminescent core” and similar termsindicate that the particles and/or cores luminesce under excitation withespecially UV and/or blue radiation (light source radiation, see below).Herein, also the term “luminescent particle” may be used to refer to the“luminescent core”. Moreover, herein also the coated luminescentparticles may be referred to as “luminescent particles”. It will beclear from the context whether the term “luminescent particle” refers toa core that is not coated or e.g. that it refers to the luminescentparticle comprising the hybrid coating, or to a luminescent particlecomprising only one or more layers of the hybrid coating.

The luminescent core (before applying the ALD coating process)especially comprises a primer layer on (a surface of) the luminescentcore. Herein, the luminescent core comprising the primer layer (on theluminescent core) is also referred to as a “primer layer comprisingluminescent core”. In embodiments, the virgin (core) material (already)comprises the primer layer. In embodiments, e.g., the core may comprisean oxide-containing surface. In further embodiments, the primer layermay be provided to the virgin (core) material, especially with themethod of the invention. Hence, in further embodiments, the method maycomprise providing a primer layer onto a core(, to provide theluminescent core comprising the primer layer on the luminescent core)(see further below).

The primer layer not necessarily is entirely conformal with the core.The primer layer may especially be evenly distributed over (the surfaceof) the luminescent core. However, the primer layer, may in embodimentsnot entirely cover the surface of the core. The primer layer may inembodiments cover the core for at least 50%, especially at least 75%,such as at least 90%, or especially at least 95% or even more especiallyat least 99%, of the surface of the core (see further below). The primerlayer may especially be configured to facilitate the deposition of themain ALD coating layer. The primer layer may function as a nucleationlayer or a seed layer for the main ALD coating layer.

The main ALD coating layer is provided onto the primer layer. Hence, inembodiments the ALD coating layer may contact the surface of theluminescent core at first locations of the luminescent core comprisingthe primer layer, and the ALD coating layer may contact the primer layerat further locations. The main ALD coating layer may optionally includea multilayer. However, the multilayers of the main ALD coating layer areall ALD layers. Therefore, this layer is indicated as (main) ALD(coating) layer (thus optionally including an ALD multilayer).Especially the main ALD coating layer comprises a multilayer with two ormore layers (having different chemical compositions), see also below.The main ALD coating layer especially at least includes one or morealuminum oxide (especially Al₂O₃) coating layers.

Likewise, the main sol-gel coating layer may optionally include amultilayer. However, the (multi-)layers of the main sol-gel coatinglayer are all sol-gel layers. Therefore, this coating layer is hereinalso indicated as a (main) sol-gel (coating) layer (thus optionallyincluding a sol-gel multilayer). Further, especially the main sol-gelcoating layer is provided on the main ALD coating layer, without anintermediate layer. The main sol-gel coating layer especially comprisessilicon oxide (especially SiO₂). An example of a multilayer may e.g.include a SiO₂—Al₂O_(3-x)(OH)_(2x) (sol-gel) multilayer (wherein 0≤x≤3),such as a stack of three or more (sol-gel) layers wherein SiO₂ andAl₂O_(3-x)(OH)_(2x) (with 0≤x≤3) alternate. Optionally on the mainsol-gel coating layer a further coating layer may be provided (seefurther below).

Especially, both the main ALD coating layer and the main sol-gel coatinglayer independently comprise metal oxides, though optionally alsohydroxides may be included in the one or more of these layers. Further,independently the main ALD coating layer and the main sol-gel coatinglayer may include mixed oxide layers. Further, the coating layers neednot necessarily to be fully stoichiometric oxides, as is known in theart.

In embodiments, the primer layer (also) comprises a sol-gel coatinglayer (provided by application of a sol-gel process). Herein, suchsol-gel coating layer may be indicated as a primary sol-gel coatinglayer, especially to distinguish from the main sol-gel coating layer.The primary sol-gel coating layer may in embodiments comprise metaloxides, and optionally hydroxides, as described herein in relation tothe main sol-gel coating layer. Further, especially, the primary sol-gelcoating layer may be provided as described in relation to the mainsol-gel coating layer, see also below, further describing the sol-gelprocess. The primary sol-gel coating layer may especially be provided(and comprise a composition) as described in relation with the mainsol-gel coating.

The primer layer may in further embodiments (also) (further) comprise anoxide-containing layer. In specific embodiments, the oxide-containinglayer is provided by application of a chemical washing process onto theluminescent core (see further below). The chemical washing process mayespecially provide a washing result layer onto the luminescent core.Hence, in embodiments, the washing result layer comprises theoxide-containing layer. The primer layer may especially function as anucleation layer or a seed layer for the main ALD coating layer. Yet,the primer layer may be structurally different for various embodiments.As described above, in embodiments, the primer layer comprises,especially consist of an oxide-containing layer (or oxide-rich layer)(at the surface of the luminescent core). In further embodiments, theprimer layer comprises, especially consist of the washing result layer.In further embodiments, the primer layer comprises, especially consistsof the primary sol-gel coating layer. In further specific embodiments,the primer layer comprises the washing result layer and the primarysol-layer.

Especially (if the core is subjected to the chemical washing process),the primary sol-gel coating layer is provided after the chemical washingprocess, and especially the primary sol-gel coating may be provided onthe washing result layer (especially the oxide-containing layer). Yet,in such embodiments, the primary sol-gel coating layer may contact thewashing result layer at a first location of the luminescent core. Theprimary sol-gel coating may contact the surface of the luminescent coreat other locations of the luminescent core. Hence, in embodiments, theprimer layer comprises an oxide-containing layer and a primary sol-gellayer, especially wherein the oxide-containing layer is arranged at asurface of the core (and at least part of the primary sol-gel coatinglayer is arranged at the oxide-containing layer).

As is described above, in embodiments, locations of the core may not becovered by the primary layer (especially the one or more of theoxide-containing layer and the primary sol-gel coating layer). Hence, inembodiments, the main ALD coating layer may (be provided to) contact theprimary sol-gel coating layer at some locations of the luminescent coreand the main ALD coating layer may (be provided to) contact the surfaceof the core at some other locations of the luminescent particle. In yetfurther embodiments, the main ALD coating may (also) (be provided to)contact the washing result layer at some further locations of theluminescent particle.

In general, the thickness of the primer layer is smaller than thethickness of the main sol-gel layer, and especially also smaller thanthe thickness of the main ALD coating layer. Further, especially themain sol-gel coating layer thickness is generally larger than the ALDcoating layer thickness. The primer layer thickness is especially equalto or smaller than 10 nm, such as equal to or smaller than 7 nm,especially equal to or smaller than 5 nm, even more especially equal toor smaller than 4 nm. The primer layer thickness may in embodiments beat least 0.1 nm, such as at least 0.2 nm, especially at least 0.5 nm,such as especially at least 1 nm. The primary layer thickness mayespecially be the result of the thickness of the primary sol-gel coatinglayer. Hence, especially the primary sol-gel coating layer may be equalto or smaller than 10 nm, such as equal to or smaller than 7 nm,especially equal to or smaller than 5 nm, such as equal to or smallerthan 4 nm. The oxide-containing layer may in embodiments be less than 1nm thick. In embodiments the primer layer has a primer layer thickness(d1) in the range of 0.1-5 nm. In further embodiments, the primer layercomprises a primary sol-gel layer provided by application of a primarysol-gel coating process. The thickness of main sol-gel coating layer maybe at least 10 times, such as at least 50 times, especially at least 100times thicker than the thickness of the primary layer.

Further, especially the main sol-gel coating layer thickness isgenerally larger than the main ALD coating layer thickness, such as atleast 1.2, like at least 1.5, like at least 2 times larger, or even atleast 4 times or at least 5 times or at least 10 times larger (than themain ALD coating layer thickness).

In specific embodiments, the method of the invention comprises (i)providing the primer layer, especially having a primer layer thickness(d1) in the range of 0.1-10 nm, especially 0.1-7 nm, such as 0.1-5 nm or0.1-4 nm, onto the core (to provide the primer layer comprisingluminescent core); (ii) providing the main ALD coating layer having amain ALD coating layer thickness (d2) in the range of 3-250 nm, such as5-250 nm, especially 5-100 nm, even more especially 5-50 nm, such asespecially 10-50 nm, even more especially 20-50 nm, onto the primerlayer (especially onto the primer layer comprising luminescent core) byapplication of the main atomic layer deposition process; and especially(iii) providing the main sol-gel coating layer having a main sol-gelcoating layer thickness (d3) in the range of 50-700 nm, such as 50-600nm, especially 75-500 nm, such as especially 100-500 nm onto the mainALD coating layer, by application of the main sol-gel process.

Especially, the primer layer has a primer layer thickness (d1) in therange of 0.1-10 nm, especially 0.1-7 nm, such as 0.1-5 nm or 0.1-4 nm.Further, especially the main ALD coating layer has a main ALD coatinglayer thickness (d2) in the range of 3-250 nm, such as 5-250 nm,especially 5-100 nm, even more especially 5-50 nm, such as especially10-50 nm, even more especially 20-50 nm. Especially, the main sol-gelcoating layer has a main sol-gel coating layer thickness (d3) in therange of 50-700 nm, such as 50-600 nm, especially 75-500 nm, such asespecially 100-500 nm.

Hence, as indicated above, the luminescent particle comprises inembodiments a luminescent core, a primer layer having a primer layerthickness (d1) in the range of 0.1-10 nm, especially 0.1-7 nm, such as0.1-5 nm or 0.1-4 nm, a main ALD coating layer having a main ALD coatinglayer thickness (d2) in the range of 3-250 nm, such as 5-250 nm,especially 5-100 nm, even more especially 5-50 nm, such as especially10-50 nm, even more especially 20-50 nm, and a main sol-gel coatinglayer having a main sol-gel coating layer thickness (d3) in the range of50-700 nm, such as 50-600 nm, especially 75-500 nm, such as especially100-500 nm.

In embodiments, the primer layer at least partly encapsulates thesurface of the luminescent core. In further embodiments, the main ALDcoating encapsulates the primer layer. In further embodiments the mainsol-gel coating encapsulates the main ALD coating layer. In yet furtherembodiments, a further ALD coating layer encapsulates the main sol-gelcoating layer (see below). Hence, the hybrid coating may comprise a mainALD coating layer and a main sol-gel coating layer, especially a primerlayer, a main ALD coating layer, and a main sol-gel coating layer. Theprimer layer is especially arranged between the surface of theluminescent core and the main ALD coating layer. The main ALD coatinglayer is especially arranged between the main sol-gel coating layer andthe primer layer.

In yet further embodiments, the luminescent particle may comprise afurther coating layer arranged on the main sol-gel coating layer. Infurther embodiments, the hybrid coating further comprises the furthercoating layer arranged at the main-sol-gel coating layer. The furthercoating layer may especially comprise a further ALD coating layer,especially encapsulating the main sol-gel coating. Hence, inembodiments, the luminescent particle (further) comprises a further ALDcoating layer arranged onto the main sol-gel coating layer. The furtherALD coating layer especially has a further ALD coating layer thickness(d4) in the range of 1-100 nm, such as 5-75 nm, especially 10-75 nm,such as especially 10-50 nm. Further, especially the further ALD coatinglayer has a chemical composition differing from the chemical compositionof the main sol-gel coating layer.

Hence, in specific embodiments, the method further comprises (iv)providing a further ALD coating layer onto the main sol-gel coating byapplication of a further atomic layer deposition process (especiallythereby providing a further ALD coated luminescent particle), especiallywherein the further ALD coating layer has a further ALD coating layerthickness (d4) in the range of 1-100 nm, such as 5-75 nm, especially10-75 nm, such as especially 10-50 nm, and especially wherein thefurther ALD coating layer has a chemical composition differing from thechemical composition of the main sol-gel coating layer. The further ALDcoating layer may be provided by an ALD process described herein,especially in relation to the main ALD layer. The further ALD layer may(also) comprise a multilayer. The further ALD layer may furtherespecially comprise a composition (and/or the (metal) oxides) describedin relation to the main ALD layer. In embodiments, the further ALDcoating layer comprises one or more oxides of one or more of Al, Zn, Hf,Ta, Zr, Ti, Sn, Nb, Y, Ga, and V, and optionally Si.

Herein the term “thickness” is used in relation to the coatings andlayers. The term especially relates to the average thickness of thecoating over the entire surface being coated by the respective layer.For instance, the primary layer may not completely cover the surface ofthe core and the (local) thickness of primer layer may be substantiallyzero at locations of the surface core. At other locations of thesurface, the maximum (local) thickness of the primer layer may be 3 nm.Then, e.g., the primer layer thickness may be in the range of largerthan 0 and smaller than 3 nm. Moreover, also if the (coating) layercompletely covers the core or another coating layer, locally thethickness may vary. Especially, e.g., the sol-gel process may providecoating layers having a somewhat pocked shape or e.g. may comprise oneor more little pinholes. Hence, the layer thicknesses described hereinare especially average layer thicknesses. However especially, at leastfor the primer layer, the main sol gel coating layer, the main ALDcoating layer and the further ALD coating layer (when present), at least50%, even more especially at least 80%, of the area of the respectivelayers have such indicated layer thickness. Especially, this indicatesthat under at least 50% of the area of such layer, such thickness willbe found.

The luminescent core of interest may in principle include each type of(virgin) luminescent particle or particulate material. However,especially of interest are those type of luminescent particulatematerials (particles) that may be less stable in air or water or a humidenvironment, such as e.g. (oxo)sulfides, (oxo)nitrides, etc. Hence, inembodiments the luminescent core (and the luminescent particlecomprising the luminescent core) comprises one or more of a nitrideluminescent material, an oxonitride luminescent material, a halogenideluminescent material, an oxohalogenide luminescent material, a sulfideluminescent material, and an oxosulfide luminescent material.Additionally or alternatively, the luminescent core (luminescentparticle) may comprise a selenide luminescent material. Hence, the term“luminescent core” (and also “luminescent particle”) may also refer to acombination of particulate materials of different types of luminescentmaterials. The luminescent core may in embodiments especially comprise aplurality of particulate luminescent materials/luminescent particles.

In a specific embodiment, the luminescent core (or the material of theluminescent core) may be selected from the following group ofluminescent material systems: MLiAl₃N₄:Eu (M=Sr, Ba, Ca, Mg),MLi₂Al₂O₂N₂:Eu (M=Ba, Sr, Ca), M₂SiO₄:Eu (M=Ba, Sr, Ca),MSe_(1-x)S_(x):Eu (M=Sr, Ca, Mg), MA₂S₄:Eu (M=Sr, Ca, A=Al, Ga)),M₂SiF₆:Mn (M=Na, K, Rb), MSiAlN₃:Eu (M=Ca, Sr), M₈ ^(Mg)(SiO₄)₄Cl₂:Eu(M=Ca, Sr), M₃MgSi₂OS:Eu (M=Sr, Ba, Ca), MSi₂O₂N₂:Eu (M=Ba, Sr, Ca),MLi₃SiO₄:Eu (M=Li, Na, K, Rb, Cs), M₂Si_(5-x)Al_(x)O_(x)N_(8-x):Eu(M=Sr, Ca, Ba). However, other systems may also be of interested toprotect by the hybrid coating. Also combinations ofparticles/particulate materials of two or more different luminescentmaterials may be applied, such as e.g. a green or a yellow luminescentmaterial in combination with a red luminescent material.

Terms like “M=Sr, Ba, Ca, Mg” indicate, as known in the art, that Mincludes one or more of Sr, Ba, Ca, and Mg. For instance, referring toMSiAlN₃:Eu (M=Ca, Sr), this may refer by way of examples to CaSiAlN₃:Eu,or to SrSiAlN₃:Eu, or to Ca_(0.8)Sr_(0.2)SiAlN₃:Eu, etc. etc. Further,the formula “MLiAl₃N₄:Eu (M=Sr, Ba, Ca, Mg),” is equal to the formula(Sr,Ba,Ca,Mg)LiAl₃N₄:Eu. Further, for clarity reasons also the formula“(M1)LiAl₃N₄:Eu, with M1=Sr, Ba, Ca” and the like may be used, e.g.,when more than one group is indicated with different elements, forinstance in a sentence like “wherein the luminescent material isselected from a group consisting of (M1)Li_(d)Mg_(a)Al_(b)N₄:Eu, with0≤a≤4; 0≤b≤4; 0≤d≤4, and M1 comprising one or more of the groupconsisting of Ca, Sr, and Ba; and(M2)Li₂Al_(2-z)Si_(z)O_(2-z)N_(2+z):Eu, wherein 0≤z≤0.1, and M2comprising one or more of the group consisting of Sr and Ba”. Further,also M1, M2, et cetera may refer to one or more of the (respective)elements. In the above given example, e.g. M1 may in embodiments be Sr,or Ba, or Ca. In further embodiments M1 may be combination of Sr and Ba,or e.g. Sr and Ca, or Sr, Ba, and Ca, etc. Moreover, the elements mayespecially be present in any ratio, e.g. 20% Sr, 20% Ca and 50%. Ba, or10% Ba and 90% Sr, etc. Likewise this applies to the other hereinindicated formulas of inorganic luminescent materials.

In further specific embodiments, the luminescent core may be selectedfrom the following group of luminescent material systems:M_(1-x-y-z)Z_(z)A_(a)B_(b)C_(c)D_(d)E_(e)N_(4-n)O_(n):ES_(x),RE_(y),with M=selected from a group consisting of Ca (calcium), Sr (strontium),and Ba (barium); Z selected from a group consisting of monovalent Na(sodium), K (potassium), and Rb (rubidium); A=selected from a groupconsisting of divalent Mg (magnesium), Mn (manganese), Zn (zinc), and Cd(cadmium) (especially, A=selected from a group consisting of divalent Mg(magnesium), Mn (manganese), and Zn (zinc), even more especiallyselected from a group consisting of divalent Mg (magnesium), Mn(manganese); B=selected from a group consisting of trivalent B (boron),Al (aluminum) and Ga (gallium); C=selected from a group consisting oftetravalent Si (silicon), Ge (germanium), Ti (titanium) and Hf(hafnium); D selected from a group consisting of monovalent Li(lithium), and Cu (copper); E selected for the group consisting of P(the element phosphor), V (vanadium), Nb (niobium), and Ta (tantalum);ES=selected from a group consisting of divalent Eu (europium), Sm(samarium) and ytterbium, especially selected from a group consisting ofdivalent Eu and Sm; RE=selected from a group consisting of trivalent Ce(cerium), Pr (praseodymium), Nd (neodymium), Sm (samarium), Eu(europium), Gd (gadolinium), Tb (terbium), Dy (dysprosium), Ho(holmium), Er (erbium), and Tm (thulium); with 0≤x≤0.2; 0≤y≤0.2;0<x+y≤0.4; 0≤z<1; 0≤n≤0.5; 0≤a≤4 (such as 2≤a≤3); 0≤b≤4; 0≤c≤4; 0≤d≤4;0≤e≤4; a+b+c+d+e=4; and 2a+3b+4c+d+5e=10−y−n+z. Especially, z≤0.9, suchas z≤0.5. Further, especially x+y+z≤0.2.

The equations a+b+c+d+e=4; and 2a+3b+4c+d+5e=10−y−n+z, respectively,especially determine the Z, A, B, C, D and E cations and O and N anionsin the lattice and thereby define (also) the charge neutrality of thesystem. For instance, the charge compensation is covered by the formula2a+3b+4c+d+5e=10−y−n+z. It covers e.g. charge compensation by decreasingO content or charge compensation by substituting a C cation by a Bcation or a B cation by an A cation, etc. For example: x=0.01, y=0.02,n=0, a=3; then 6+3b+4c=10−0.02; with a+b+c=4; b=0.02, c=0.98.

As will be clear to a person skilled in the art, a, b, c, d, e, n, x, y,z are always equal to or larger than zero. When a is defined incombination with the equations a+b+c+d+e=4; and 2a+3b+4c+d+5e=10−y−n+z,then in principle, b, c, d, and e do not need to be defined anymore.However, for the sake of completeness, herein also 0≤b≤4; 0≤c≤4; 0≤d≤4;0≤e≤4 are defined.

Assume a system like SrMg₂Ga₂N₄:Eu. Here, a=2, b=2, c=d=e=y=z=n=0. Insuch system, 2+2+0+0+0=4 and 2*2+3*2+0+0+0=10−0−0+0=10. Hence, bothequations are complied with. Assume that 0.5 O is introduced. A systemwith 0.5 O can e.g. be obtained when 0.5 Ga—N is replaced by 0.5 Mg—O(which is a charge neutral replacement). This would result inSrMg_(2.5)Ga_(1.5)N_(3.5)O_(0.5):Eu. Here, in such system2.5+1.5+0+0+0=4 and 2*2.5+3*1.5+0+0+0=10−0−0.5+0=9.5. Hence, also hereboth equations are complied with.

As indicated above, in advantageous embodiments d>0 and/or z>0,especially at least d>0. Especially, the phosphor comprises at leastlithium. In yet another embodiment, 2≤a≤3, and especially also d=0, e=0and z=0. In such instances, the phosphor is amongst others characterizedby a+b+c=4; and 2a+3b+4c=10−y−n.

In a further specific embodiment, which may be combined with the formerembodiments e=0. In yet a further specific embodiment, which may becombined with the former embodiments, M is Ca and/or Sr.

Hence, in a specific embodiment, the phosphor has the formula M(Caand/or Sr)_(1-x-y)Mg_(a)Al_(b)Si_(c)N_(4-n)O_(n):ES_(x),RE_(y) (I), withES=selected from a group consisting of divalent Eu (europium) or Sm(samarium) or Yb (ytterbium); RE=selected from a group consisting oftrivalent Ce (cerium), Pr (praseodymium), Nd (neodymium), Sm (samarium),Eu (europium), Gd (gadolinium), Tb (terbium), Dy (dysprosium), Ho(holmium), Er (erbium), and Tm (thulium), wherein y/x<0.1, especially<0.01, and n≤0.1, especially <0.01, even more especially <0.001, yeteven more especially <0.0001. Hence, in this embodiment, substantiallysamarium and or europium containing phosphors are described. Forinstance, when divalent Eu is present, with x=0.05, and for instance y1for Pr may be 0.001, and y2 for Tb may be 0.001, leading to any=y1+y2=0.002. In such instance, y/x=0.04. Even more especially, y=0.However, as indicated elsewhere when Eu and Ce are applied, the ratioy/x may be larger than 0.1.

The condition 0<x+y≤0.4 indicates that M may be substituted with intotal up to 40% of ES and/or RE. The condition “0<x+y≤0.4” incombination with x and y being between 0 and 0.2 indicates that at leastone of ES and RE are present. Not necessarily both types are present. Asindicated above, both ES and RE may each individually refer to one ormore subspecies, such as ES referring to one or more of Sm and Eu, andRE referring to one or more of Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er,and Tm.

Especially, when europium is applied as divalent luminescent species ordopant (i.e. Eu²⁺), the molar ratio between samarium and europium(Sm/Eu) is <0.1, especially <0.01, especially <0.001. The same applieswhen europium in combination with ytterbium would be applied. Wheneuropium is applied as divalent luminescent species or dopant, the molarratio between ytterbium and europium (Yb/Eu) is <0.1, especially <0.01,especially <0.001. Would all three together be applied, then the samemolar ratios might apply, i.e. ((Sm+Yb)/Eu) is <0.1, especially <0.01,especially <0.001.

Especially, x is in the range of 0.001-0.2 (i.e. 0.001≤x≤0.2), like0.002-0.2, such as 0.005-0.1, especially 0.005-0.08. Especially in thecase of divalent Europium in the herein described systems, the molarpercentage may be in the range of 0.1-5% (0.001≤x≤0.05), such as 0.2-5%,like 0.5-2%. For other luminescent ions, x may (but is not necessarily)in embodiments be equal to or larger than 1% (x equal to or larger than0.01).

In a specific embodiment, the phosphor is selected from a groupconsisting of (Sr,Ca)Mg₃SiN₄:Eu, (Sr,Ca)Mg₂Al₂N₄:Eu, (Sr,Ca)LiAl₃N₄:Euand (Sr,Ca)Li_(d)Mg_(a)Al_(b)N₄:Eu, with a, b, d as defined above.

As also indicated herein, the notation “(Sr,Ca)”, and similar notationswith other elements, indicates that the M-positions are occupied with Srand/or Ca cations (or other elements, respectively).

In a further specific embodiments the phosphor is selected from a groupconsisting of Ba_(0.95)Sr_(0.05)Mg₂Ga₂N₄:Eu, BaMg₂Ga₂N₄:Eu,SrMg₃SiN₄:Eu, SrMg₂Al₂N₄:Eu, SrMg₂Ga₂N₄:Eu, BaMg₃SiN₄:Eu, CaLiAl₃N₄:Eu,SrLiAl₃N₄:Eu, CaLi_(0.5)MgAl_(2.5)N₄:Eu, and SrLi_(0.5)MgAl_(2.5)N₄:Eu.Further (non-limiting) examples for such phosphors are e.g.(Sr_(0.5)Ca_(0.2))_(0.995)LiAl_(2.91)Mg_(0.09)N_(3.91)O_(0.09):Eu_(0.005);(Sr_(0.9)Ca_(0.1))_(0.905)Na_(0.09)LiAl₃N_(3.91)O_(00.9):Eu_(0.005);(Sr_(0.8)Ca_(0.03)Ba_(0.17))_(0.989)LiAl_(2.99)Mg_(0.01)N₄:Ce_(0.01),Eu_(0.001);Ca_(0.995)LiAl_(2.995)Mg_(0.005)N_(3.995)O_(0.005):Yb_(0.005) (YB(II));Na_(0.995)MgAl₃N₄:Eu_(0.005);Na_(0.895)Ca_(0.1)Mg_(0.9)Li_(0.1)Al₃N₄:Eu_(0.005);Sr_(0.99)LiMgAlSiN₄:Eu_(0.01);Ca_(0.995)LiAl_(2.955)Mg_(0.045)N_(3.96)O_(0.04):Ce_(0.005);(Sr_(0.9)Ca_(0.1))_(0.998)Al_(1.99)Mg_(2.01)N_(3.99)O_(0.01):Eu_(0.002);(Sr_(0.9)Ba_(0.1))_(0.998)Al_(1.99)Mg_(2.01)N_(3.99)O_(0.01):Eu_(0.002).

In a further specific embodiment, the phosphor is selected from a groupconsisting of (Sr,Ca)Mg₃SiN₄:Eu and (Sr,Ca)Mg₂Al₂N₄:Eu. In yet anotherspecific embodiment, the phosphor is selected from a group consisting ofBa_(0.95)Sr_(0.05)Mg₂Ga₂N₄:Eu, BaMg₂Ga₂N₄:Eu, SrMg₃SiN₄:Eu,SrMg₂Al₂N₄:Eu, SrMg₂Ga₂N₄:Eu, and BaMg₃SiN₄:Eu. Especially, thesephosphors, and even more especially (Sr,Ca)Mg₃SiN₄:Eu and(Sr,Ca)Mg₂Al₂N₄:Eu may be phosphors having good luminescent properties,amongst others in terms of spectral position and distribution of theluminescence.

In further specific embodiments the phosphor, especially the luminescentmaterial, is selected from a group consisting of (Sr,Ca)LiAl₃N₄:Eu and(Sr,Ca,Ba)Li_(d)Mg_(a)Al_(b)N₄:Eu, with 0≤a≤4; 0≤b≤4; 0≤d≤4; and a+b+d=4and 2a+3b+d=10. In yet another specific embodiment, the phosphor isselected from a class of (Sr,Ba)Li₂Al_(2-z)Si_(z)O_(2-z)N_(2+z):Eu with0≤z≤0.1.

The luminescent material is in embodiments selected from a groupSrLiAl₃N₄:Eu. The luminescent material may e.g. comprise SrLiAl₃N₄:Euwith an Eu doping concentration in the range 0.1-5%, especially 0.1-2%,such as 0.2-1.2% relative to Sr.

In further specific embodiments, the phosphor/luminescent core (theluminescent material) comprisesSrLi₂Al_(1.995)Si_(0.005)O_(1.995)N_(2.005):Eu²⁺, especially with an Eudoping concentration in the range 0.1-5%, especially 0.1-2%, 0.2-1.5%relative to Sr.

Of especial interest are phosphors wherein the phosphor complies with0≤x≤0.2, y/x<0.1, M comprises at least Sr, z≤0.1, a≤0.4, 2.5≤b≤3.5, Bcomprises at least Al, c≤0.4, 0.5≤d≤1.5, D comprises at least Li, e≤0.4,n≤0.1, and wherein ES at least comprises Eu. Especially, y+z≤0.1.Further, especially x+y+z≤0.2. Further, especially a is close to 0 orzero. Further, especially b is about 3. Further, especially c is closeto 0 or zero. Further, especially d is about 1. Further, especially e isclose to 0 or zero. Further, especially n is close to 0 or zero.Further, especially y is close to 0 or zero. Especially good systems, interms of quantum efficiency and hydrolysis stability are those withz+d>0, i.e. one or more of Na, K, Rb, Li and Cu(I) are available,especially at least Li, such as e.g. (Sr,Ca)LiAl₃N₄:Eu and(Sr,Ca)Li_(d)Mg_(a)Al_(b)N₄:Eu, with a, b, d as defined above. Infurther specific embodiments the phosphor is selected from a groupconsisting of CaLiAl₃N₄:Eu, SrLiAl₃N₄:Eu, CaLi_(0.5)MgAl_(2.5)N₄:Eu, andSrLi_(0.5)MgAl_(2.5)N₄:Eu. Further phosphors of special interest are(Sr,Ca,Ba)(Li,Cu)(Al,B,Ga)₃N₄:Eu, which comprises as M ion at least Sr,as B ion at least Al, and as D ion at least Li.

In embodiments, the phosphor (the luminescent core) is selected from agroup consisting of (M1)Li_(d)Mg_(a)Al_(b)N₄:Eu, with 0≤a≤4; 0≤b≤4;0≤d≤4, and M1 comprising one or more (elements selected) from a groupconsisting of Ca, Sr, and Ba; and a+b+d=4 and 2a+3b+d=10; and(M2)Li₂Al_(2-z)Si_(z)O_(2-z)N_(2+z):Eu, wherein 0≤z≤0.1, and M2comprising one or more (elements selected) from the group consisting ofSr and Ba.

Hence, in a specific embodiment, the luminescent particles comprise aluminescent material selected from (the) SrLiAl₃N₄:Eu²⁺ (class). Theterm “class” herein especially refers to a group of materials that havethe same crystallographic structure(s). Further, the term “class” mayalso include partial substitutions of cations and/or anions. Forinstance, in some of the above-mentioned classes Al—O may partially bereplaced by Si—N (or the other way around). The class of SrLiAl₃N₄:Eu²⁺may especially relate to a group of materials that have the samecrystallographic structure, especially wherein Sr is partially replacedby divalent Eu, e.g. by 0.1% or 2%. For instanceSr_(0.995)LiAl₃N₄:Eu_(0.005) and Sr_(0.98)LiAl₃N₄:Eu_(0.02) are elementsof such class. Likewise theSrLi₂Al_(1.995)Si_(0.005)O_(1.995)N_(2.005):Eu²⁺ class (see also below)may e.g. compriseSr_(0.999)Li₂Al_(1.995)Si_(0.005)O_(1.995)N_(2.005):Eu_(0.001) andSr_(0.955)Li₂Al_(1.995)Si_(0.005)O_(1.995)N_(2.005):Eu_(0.015).Optionally also part of Sr may be replaced by another alkaline earthmetal (group 2 elements of the periodic table). Examples of theSrLiAl₃N₄:Eu²⁺ class are provided above. However, other luminescentmaterials may thus also be possible.

In further embodiments, the luminescent material (or phosphor) isselected from a group consisting of (Sr,Ca)LiAl₃N₄:Eu,(Sr,Ca,Ba)Li_(d)Mg_(a)Al_(b)N₄:Eu, with 0≤a≤4; 0≤b≤4; 0≤d≤4; and a+b+d=4and 2a+3b+d=10, and (Sr,Ba)Li₂Al_(2-z)Si_(z)O_(2-z)N_(2+z):Eu, wherein0≤z≤0.1.

The luminescent core may thus especially comprise a phosphor. Moreover,the luminescent core especially comprises a luminescent materialdescribed herein, especially in relation to the phosphor. The method maybe applied for providing more than one, especially a plurality ofluminescent particles with a hybrid coating (and especially coating morethan one luminescent core).

In further embodiments of the luminescent material, the luminescentcore, comprises a (phosphor) material selected from a group consistingof (i) (the) SrLiAl₃N₄:Eu²⁺ (class), especially wherein an (Eu) dopingconcentration is in the range of 0.1-5%, especially 0.1-2%, even moreespecially 0.2-1.2%, relative to Sr, and (ii) (the)SrLi₂Al_(1.995)Si_(0.005)O_(1.995)N_(2.005):Eu²⁺ (class), especiallywherein the Eu doping concentration is in the range of 0.1-5%,especially 0.1-2%, even more especially 0.2-1.5 at. % relative to Sr.Further, especially, the third coating layer comprises SiO₂, and one ormore layers of the multilayer comprise one or more of Ta₂O₅, HfO₂, TiO₂and ZrO₂ and wherein one or more (other) layers of the multilayercomprise Al₂O₃, especially herein the layer contacting the main sol-gelcoating layer consist of one or more metal oxides selected from a groupof HfO₂, ZrO₂, TiO₂, Ta₂O₅.

Such luminescent particles may have a number averaged particle size inthe range of 0.1-50 μm, such as in the range of 0.5-40 μm, such asespecially in the range of 0.5-20 μm. Hence, the luminescent core mayhave dimensions such as at maximum about 500 μm, such as at maximum 100μm, like at maximum about 50 μm. especially with the larger particlesizes, substantially only individual particles may be coated, leadingthus to luminescent core dimensions in the order of 50 μm or smaller.Hence, the invention is directed to the coating of particles. Thedimensions of the luminescent core may substantially be smaller whennanoparticles or quantum dots are used as basis for the particulateluminescent material. In such instance, the cores may be smaller thanabout 1 μm or substantially smaller (see also below for the dimensionsof the QDs).

Alternatively or additionally, the luminescent particle(s), especiallythe luminescent core(s), include luminescent quantum dots. The term“quantum dot” or “luminescent quantum dot” may in embodiments also referto a combination of different type of quantum dots, i.e. quantum dotsthat have different spectral properties. The QDs are herein alsoindicated as “wavelength converter nanoparticles” or “luminescentnanoparticles”. The term “quantum dots” especially refer to quantum dotsthat luminesce in one or more of the UV, visible and IR (upon excitationwith suitable radiation, such as UV radiation). The quantum dots orluminescent nanoparticles, which are herein indicated as wavelengthconverter nanoparticles, may for instance comprise group II-VI compoundsemiconductor quantum dots selected from a group consisting of(core-shell quantum dots, with the core selected from a group consistingof) CdS, CdSe, CdTe, ZnS, ZnSe, ZnTe, HgS, HgSe, HgTe, CdSeS, CdSeTe,CdSTe, ZnSeS, ZnSeTe, ZnSTe, HgSeS, HgSeTe, HgSTe, CdZnS, CdZnSe,CdZnTe, CdHgS, CdHgSe, CdHgTe, HgZnS, HgZnSe, HgZnTe, CdZnSeS, CdZnSeTe,CdZnSTe, CdHgSeS, CdHgSeTe, CdHgSTe, HgZnSeS, HgZnSeTe and HgZnSTe. Inanother embodiment, the luminescent nanoparticles may for instance begroup III-V compound semiconductor quantum dots selected from a groupconsisting of (core-shell quantum dots, with the core selected from agroup consisting of) GaN, GaP, GaAs, AlN, AlP, AlAs, InN, InP, InGaP,InAs, GaNP, GaNAs, GaPAs, AlNP, AlNAs, AlPAs, InNP, InNAs, InPAs,GaAlNP, GaAlNAs, GaAlPAs, GaInNP, GaInNAs, GaInPAs, InAlNP, InAlNAs, andInAlPAs. In yet a further embodiment, the luminescent nanoparticles mayfor instance be I-III-VI2 chalcopyrite-type semiconductor quantum dotsselected from a group consisting of (core-shell quantum dots, with thecore selected from a group consisting of) CuInS₂, CuInSe₂, CuGaS₂,CuGaSe₂, AgInS₂, AgInSe₂, AgGaS₂, and AgGaSe₂. In yet a furtherembodiment, the luminescent nanoparticles may for instance be(core-shell quantum dots, with the core selected from a group consistingof) I-V-VI2 semiconductor quantum dots, such as selected from a groupconsisting of (core-shell quantum dots, with the core selected from agroup consisting of) LiAsSe₂, NaAsSe₂ and KAsSe₂. In yet a furtherembodiment, the luminescent nanoparticles may for instance be core-shellquantum dots, with the core selected from a group consisting of) group(IV-VI compound semiconductor nano crystals such as SbTe. In a specificembodiment, the luminescent nanoparticles are selected from a groupconsisting of (core-shell quantum dots, with the core selected from agroup consisting of) InP, CuInS₂, CuInSe₂, CdTe, CdSe, CdSeTe, AgInS₂and AgInSe₂. In yet a further embodiment, the luminescent nanoparticlesmay for instance be one of the group (of core-shell quantum dots, withthe core selected from a group consisting of) II-VI, III-V, I-III-V andIV-VI compound semiconductor nano crystals selected from the materialsdescribed above with inside dopants such as ZnSe:Mn, ZnS:Mn. The dopantelements could be selected from Mn, Ag, Zn, Eu, S, P, Cu, Ce, Tb, Au,Pb, Tb, Sb, Sn and Tl. Herein, the luminescent nanoparticles basedluminescent material may also comprise different types of QDs, such asCdSe and ZnSe:Mn. The luminescent core may comprise one or more,especially more, (of the same or different) (types of) luminescentnanoparticles.

It appears to be especially advantageous to use II-VI quantum dots.Hence, in embodiments the semiconductor based luminescent quantum dotscomprise II-VI quantum dots, especially selected from a group consistingof (core-shell quantum dots, with the core selected from a groupconsisting of) CdS, CdSe, CdTe, ZnS, ZnSe, ZnTe, HgS, HgSe, HgTe, CdSeS,CdSeTe, CdSTe, ZnSeS, ZnSeTe, ZnSTe, HgSeS, HgSeTe, HgSTe, CdZnS,CdZnSe, CdZnTe, CdHgS, CdHgSe, CdHgTe, HgZnS, HgZnSe, HgZnTe, CdZnSeS,CdZnSeTe, CdZnSTe, CdHgSeS, CdHgSeTe, CdHgSTe, HgZnSeS, HgZnSeTe andHgZnSTe, even more especially selected from a group consisting of CdS,CdSe, CdSe/CdS and CdSe/CdS/ZnS.

In embodiments, the wavelength converter nanoparticles have an averageparticle size in a range of about 1 to about 1000 nanometers (nm), andpreferably in a range of about 1 to about 100 nm. In embodiments,nanoparticles have an average particle size in a range of about 1 toabout 20 nm. In embodiments, nanoparticles have an average particle sizein a range of about 1 to about 10 nm. The luminescent nanoparticles(without coating) may have dimensions in the range of about 2-50 nm,such as 2-20 nm, especially 2-10 nm, even more especially 2-5 nm;especially at least 90% of the nanoparticles have dimension in theindicated ranges, respectively, (i.e. e.g. at least 90% of thenanoparticles have dimensions in the range of 2-50 nm, or especially atleast 90% of the nanoparticles have dimensions in the range of 2-5 nm).The term “dimensions” especially relate to one or more of length, width,and diameter, dependent upon the shape of the nanoparticle. Typical dotsare made of binary alloys such as cadmium selenide, cadmium sulfide,indium arsenide, and indium phosphide. However, dots may also be madefrom ternary alloys such as cadmium selenide sulfide. These quantum dotscan contain as few as 100 to 100,000 atoms within the quantum dotvolume, with a diameter of 10 to 50 atoms. This corresponds to about 2to 10 nanometers. For instance, spherical particles such as CdSe, InP,or CuInSe₂, with a diameter of about 3 nm may be provided. Theluminescent nanoparticles (without coating) may have the shape ofspherical, cube, rods, wires, disk, multi-pods, etc., with the size inone dimension of less than 10 nm. For instance, nanorods of CdSe withthe length of 20 nm and a diameter of 4 nm may be provided. Hence, inembodiments the semiconductor based luminescent quantum dots comprisecore-shell quantum dots. In yet further embodiments, the semiconductorbased luminescent quantum dots comprise dots-in-rods nanoparticles. Acombination of different types of particles may also be applied. Here,the term “different types” may relate to different geometries as well asto different types of semiconductor luminescent material. Hence, acombination of two or more of (the above indicated) quantum dots orluminescent nanoparticles may also be applied.

In embodiments, nanoparticles can comprise semiconductor nanocrystalsincluding a core comprising a first semiconductor material and a shellcomprising a second semiconductor material, wherein the shell isdisposed over at least a portion of a surface of the core. Asemiconductor nanocrystal including a core and shell is also referred toas a “core/shell” semiconductor nanocrystal. Any of the materialsindicated above may especially be used as core. Therefore, the phrase“core-shell quantum dots, with the core selected from a group consistingof” is applied in some of the above lists of quantum dot materials. Theterm “core-shell” may also refer to “core-shell-shell”, etc., includinggradient alloy shell, or dots in rods, etc.

For example, the semiconductor nanocrystal can include a core having theformula MX, where M can be cadmium, zinc, magnesium, mercury, aluminum,gallium, indium, thallium, or mixtures thereof, and X can be oxygen,sulfur, selenium, tellurium, nitrogen, phosphorus, arsenic, antimony, ormixtures thereof. Examples of materials suitable for use assemiconductor nanocrystal cores include, but are not limited to, ZnO,ZnS, ZnSe, ZnTe, CdO, CdS, CdSe, CdTe, MgS, MgSe, GaAs, GaN, GaP, GaSe,GaSb, HgO, HgS, HgSe, HgTe, InAs, InN, InP, InGaP, InSb, AlAs, AlN, AlP,AlSb, TIN, TIP, TlAs, TlSb, PbO, PbS, PbSe, PbTe, Ge, Si, an alloyincluding any of the foregoing, and/or a mixture including any of theforegoing, including ternary and quaternary mixtures or alloys.

The shell can be a semiconductor material having a composition that isthe same as or different from the composition of the core. The shellcomprises an overcoat of a semiconductor material on a surface of thecore semiconductor nanocrystal can include a Group IV element, a GroupII-VI compound, a Group II-V compound, a Group III-VI compound, a GroupIII-V compound, a Group IV-VI compound, a Group 1-III-VI compound, aGroup II-IV-VI compound, a Group II-IV-V compound, alloys including anyof the foregoing, and/or mixtures including any of the foregoing,including ternary and quaternary mixtures or alloys. Examples include,but are not limited to, ZnO, ZnS, ZnSe, ZnTe, CdO, CdS, CdSe, CdTe, MgS,MgSe, GaAs, GaN, GaP, GaSe, GaSb, HgO, HgS, HgSe, HgTe, InAs, InN, InP,InGaP, InSb, AlAs, AlN, AlP, AlSb, TIN, TIP, TlAs, TlSb, PbO, PbS, PbSe,PbTe, Ge, Si, an alloy including any of the foregoing, and/or a mixtureincluding any of the foregoing. For example, ZnS, ZnSe or CdSovercoatings can be grown on CdSe or CdTe semiconductor nanocrystals. Anovercoating process is described, for example, in U.S. Pat. No.6,322,901. By adjusting the temperature of the reaction mixture duringovercoating and monitoring the absorption spectrum of the core, overcoated materials having high emission quantum efficiencies and narrowsize distributions can be obtained. The overcoating may comprise one ormore layers. The overcoating comprises at least one semiconductormaterial which is the same as or different from the composition of thecore. Preferably, the overcoating has a thickness from about one toabout ten monolayers. An overcoating can also have a thickness greaterthan ten monolayers. In embodiments, more than one overcoating can beincluded on a core.

In embodiments, the surrounding “shell” material can have a band gapgreater than the band gap of the core material. In certain otherembodiments, the surrounding shell material can have a band gap lessthan the band gap of the core material. In embodiments, the shell can bechosen so as to have an atomic spacing close to that of the “core”substrate. In certain other embodiments, the shell and core materialscan have the same crystal structure.

Examples of semiconductor nanocrystal (core)shell materials include,without limitation: red (e.g., (CdSe)ZnS (core)shell), green (e.g.,(CdZnSe)CdZnS (core)shell, etc.), and blue (e.g., (CdS)CdZnS (core)shell(see further also above for examples of specific wavelength converternanoparticles, based on semiconductors.

Therefore, in embodiments the luminescent particle or the luminescentcore comprises a luminescent material selected from a group consistingof luminescent quantum dots comprising one or more core materialsselected from a group consisting of CdS, CdSe, ZnS, and ZnSe. Hence, inembodiments the luminescent particle or luminescent core may also beselected from a group of luminescent nanoparticles such as quantum dotsor quantum rods of composition MX (M=Cd, Zn, X=Se, S). Such particlesmay have a number averaged particle size (i.e. especiallylength/width/height, diameter), in the range of 1-50 nm.

As discussed above, the luminescent particle especially comprises a mainALD coating layer configured at the primer layer. In embodiments, theluminescent particle may further comprise a further ALD coating layerconfigured at (onto) the main sol-gel coating layers. The main ALDcoating layer may be deposited by application of the main atomic layerdeposition process (“main ALD process”). The further ALD coating layermay be deposited by application of the further atomic layer depositionprocess (“further ALD process”). The main atomic layer depositionprocess as well as the (optional) further atomic layer depositionprocess both are an atomic layer deposition process (“ALD process”). Itwill be understood that these processes may comprise the same ALDprocess. Yet, e.g., the conditions of the main ALD process may differfrom the conditions of the further ALD process. For instance, the metaloxide precursor(s) used in the main ALD process may differ from theone(s) used in the further ALD process. The duration of the depositionmay differ, the temperature may differ, etc. Yet especially the metaloxide precursor(s) that may be applied in the further ALD process may bethe metal oxide precursor(s) described in relation to the (main) ALDprocess (and vice versa).

Hence, the main ALD coating layer and the optional further ALD coatinglayer may be formed by an atomic layer deposition type process. In suchprocess a polymeric network is formed by reaction of a metal oxideprecursor with an oxygen source such as water and/or ozone in the gasphase. The ALD reaction is “splitted” in (at least) two parts. In afirst step the metal (oxide) precursor is fed into a(n ALD) reactor andadsorbs and/or reacts with reactive groups on the particle surfaces andsubstantially all non-reacted or non-adsorbed precursor molecules areremoved by reactor purging. In a second step the oxygen source is fedinto the reactor and reacts with the metal source on the particlesurfaces followed by purging of the reactor to remove substantially allremaining oxygen source molecules and hydrolysis products formed bycondensation reactions. The two steps lead to formation of an atomiclayer (or monolayer) because of the self-limiting nature of the surfacereaction. These atomic layer reaction steps may be repeated multipletimes to form the final ALD coating. The ALD process further allows itto deposit layers of different composition by consecutively feedingdifferent metal oxide precursor into the reactor to form multicomponentlayers or nanolaminates with tailored chemical, mechanical, and opticalproperties (see further below).

The term “metal oxide precursor” especially indicates a precursor of themetal oxide. The precursor itself may not be a metal oxide but may e.g.include metal organic molecule. Hence, especially the metal (oxide)precursors for ALD may typically include metal halides, alkoxides,amides, and other metal (organic) compounds. The term metal oxideprecursor may relate to more than one different metal oxide precursor,especially for more than one different metal oxides The step by stepnature of the ALD process allows to easily deposit defined layerthicknesses. The ALD process further allows it to deposit layers ofdifferent composition by consecutively feeding different metal oxideprecursors into the reactor to form multicomponent layers ornanolaminates. Hence, in a specific embodiment the main ALD coatinglayer (and/or the further ALD coating layer) comprises a multilayer (ora nanolaminate) (see also below).

For the ALD process, amongst others a fluidized bed reactor may beapplied.

Hence, in a specific embodiment the main ALD coating layer is providedby application of the (main) atomic layer deposition process. Further,in embodiments, the further ALD coating layer (also) is provided byapplication of the (further) ALD process. In embodiments, a staticpowder bed is used for ALD coating of the primer layer and/or for ALDcoating of the main sol-gel coating. However, also a fluidized bed maybe applied (for one or more of the ALD processes). Other type ofreactors may also be applied. As is described above, the primer layermay facilitate deposition of the main ALD coating layer especially byfunctioning as a nucleation layer or a seed layer for the main ALDcoating layer. Especially, reactive groups on the particle surface maybe provided by the primer layer (and also by the main sol-gel coatinglayer).

For instance, silanol groups (assuming a primary and/or main silicasol-gel coating layer) at the surface of the sol-gel coating layer actas reactive sites during ALD of the initial layers. In an embodiment,alumina is deposited by using Al(CH₃)₃ (TMA) as metal oxide precursorand (subsequently exposure to) water as the oxygen source. In the firstreaction step, TMA reacts with surface silanol groups of the silicasol-gel coating layer according to:

≡Si—OH+Al(CH₃)₃→≡Si—O—Al(CH₃)₂+CH₄

Water then reacts in the second reaction step with the metal oxideprecursor by hydrolysis followed by condensation reactions:

≡Si—O—Al(CH₃)₂+2H₂O→≡Si—O—Al(OH)₂+2CH₄

2≡Si—O—Al(OH)₂→≡Si—O—Al(OH)—O—Al(OH)—O—Si≡+H₂O

Further, particle agglomeration may substantially be prevented byapplying the primer sol-gel layer (and the main sol-gel coating layer)with a structured, nano-porous surface, such as of the silica sol-gelcoating layer (see below).

The ALD process can easily be scaled up and nearly no powder or particleloss during ALD coating is observed. Commercially available ALD reactorsfor powder coating are e.g. sold by Picosun Oy with e.g. a cartridgesample holder (POCA™). A system that may be used for the ALD process ise.g. described in WO 2013171360 A1, though other systems may also beapplied.

A (non-limited) number of suitable materials for the ALD coating layerare listed in the following table:

Oxide Oxygen Deposition material Metal (oxide) precursor source T [° C.]Al₂O₃ Al(CH₃)₃ (TMA) or HAl(CH₃)₂ H₂O or O₃ 100-400  HfO₂ Hf(N(CH₃)₂)₄or Hf(N(CH₂CH₃)₂)₄ H₂O 80-300 Ta₂O₅ TaCl₅ or Ta(N(CH₃)₂)₅ H₂O 80-300ZrO₂ ZrCl₄ or Zr(N(CH₃)₂)₄ H₂O 80-300 TiO₂ TiCl₄, Ti(OCH₃)₄ or Ti(OEt)₄H₂O 80-300 SiO₂ SiCl₄, H₂N(CH₂)₃Si(OEt)₃ or Si(OEt)₄ H₂O or O₃ 150-300 

Alternatively or additionally, niobium oxide (especially Nb₂O₅) oryttrium oxide (Y₂O₃) may be applied. Metal precursors thereof are e.g.,(tert-butylimido)-tris (diethylamino)-niobium, NbF₅, or NbCl₅, andTris(ethylcyclopentadienyl) Yttrium, respectively. In furtherembodiments zinc oxide (ZnO) may be applied. Metal precursors thereofthat e.g. may be applied are diethylzinc (DEZ), Zn(C₂H₅)₂ anddimethylzinc (DMZ) Zn(CH₃)₂. However, other materials may also beapplied. Hence, in the atomic layer deposition process a metal oxideprecursor may especially be selected from a group of metal oxideprecursors of metals comprising Al, Zn, Hf, Ta, Zr, Ti, and Sn (andoptionally Si). Alternatively or additionally, metal precursors of oneor more metals comprising Ga, Ge, V and Nb may be applied. Even moreespecially, alternating layers of two or more of these precursors areapplied, wherein at least one precursor is an Al metal oxide precursor,and another precursor is selected from a group consisting of a Hf metaloxide precursor, a Zn metal oxide precursor, a Ta metal oxide precursor,a Zr metal oxide precursor, a Ti metal oxide precursor, and a Sn metaloxide precursor, especially selected from a group consisting of a Hfmetal oxide precursor, a Ta metal oxide precursor, a Ti metal oxideprecursor, and a Zr metal oxide precursor, such as selected from a groupconsisting of a Hf metal oxide precursor, a Ta metal oxide precursor,and a Zr metal oxide precursor, even more especially a Ta metal oxideprecursor. Especially Hf, Zr, and Ta appear to provide relatively lighttransmissive layers, whereas Ti, for instance, may provide relativelyless light transmissive layers. Using TiCl₄ as the metal oxide precursor(for a TiO₂ layer) may provide a cost efficient layer. Processing withTa, Hf and Zr seems to be relatively easier than Si, for instance. Theterms “oxide precursor” or “metal oxide precursor” or “metal (oxide)precursor” may also refer to a combination of two or more chemicallydifferent precursors. These precursors especially form an oxide uponreaction with the oxygen source (and are therefore indicated as metaloxide precursor). The metal oxide precursors may in embodiments beselected independently from each other for successive ALD cycles. Forinstance, in embodiments, a ZnO layer and an Al₂O₃ layer are depositedalternately to obtain an AZO layer (Al₂O₃:ZnO, or “aluminum-doped zincoxide layer”). The AZO layer may be a conductive layer and may bedeposited using. e.g., trimethylaluminum, diethylzinc, and water as anoxygen source. In further embodiments, a (another) metal oxide is (also)deposited in multiple consecutive cycles (and optionally successively afurther metal oxide is deposited (optionally also in multipleconsecutive cycles).

Further, the term “metal oxide precursors of metals comprising Al, Zn,Hf, Ta, Zr, Ti, and Sn” and comparable terms in phrases like “in theatomic layer deposition process a metal oxide precursor is selected froma group of metal oxide precursors of metals comprising Al, Zn, Hf, Ta,Zr, Ti, and Sn” especially refers to metal oxide precursors of metalsselected from a group consisting of the given metals (in this respectAl, Zn, Hf, Ta, Zr, Ti, Sn). Furthermore, in embodiments one or moremetal oxides precursors are selected. For instance, with reference tothe list given above, the metal oxide precursor “that is selected fromthe group of metal oxide precursors of metals comprising Al, Zn, Hf, Ta,Zr, Ti, and Sn” may comprise any combination of metal oxide precursorsof two or more metals selected from the group consisting of Al, Zn, Hf,Ta, Zr, Ti, and Sn. In embodiments, e.g. the metal oxide precursorcomprises a combination of TaCl₅ and HAl(CH₃)₂. In further embodiments,the metal oxide precursor comprises only Al(CH₃)₃.

Hence, in embodiments, in the main atomic layer deposition process ametal oxide precursor is selected from a group of metal oxide precursorsof metals selected from a group consisting of Al, Zn, Hf, Ta, Zr, Ti,Sn, Nb, Y, Ga, and V (and optionally Si). Especially, in furtherembodiments, in the main atomic layer deposition process the metal oxideprecursor is selected from a group of metal oxide precursors of metalscomprising Al, Hf, Ta, Zr, and Ti (especially metal oxide precursors ofmetals selected from a group consisting of Al, Hf, Ta, Zr, and Ti). Infurther embodiments, in the main atomic layer deposition process a metaloxide precursor selected from a group consisting of Al(CH₃)₃, HAl(CH₃)₂,Hf(N(CH₃)₂)₄, Hf(N(CH₂CH₃)₂)₄, Hf[N(CH₃)(CH₂CH₃)]4, TaCl₅, Ta(N(CH₃)₂)₅,Ta{[N(CH₃)(CH₂CH₃)]3N(C(CH₃)₃)}, ZrCl₄, Zr(N(CH₃)₂)₄, TiCl₄, Ti(OCH₃)₄,and Ti(OCH₂CH₃)₄ and an oxygen source selected from a group consistingof H₂O and O₃ are applied. Additionally or alternatively, in the (main)atomic layer deposition process a metal oxide precursor is selected froma group consisting of Zn(C₂H₅)₂ and Zn(CH₃)₂. In yet furtherembodiments, additionally or alternatively, in the (main) atomic layerdeposition process, the metal precursor is selected from a groupconsisting of (tert-butylimido)-tris (diethylamino)-niobium, NbF₅,NbCl₅, and Tris(ethylcyclopentadienyl)Yttrium.

The metal oxide precursor(s) in the further atomic layer depositionprocess is especially independently selected from the metal oxideprecursor(s) in the main atomic layer deposition process and mayespecially (also) be selected from a group of metal oxide precursors ofmetals selected from a group consisting of Al, Zn, Hf, Ta, Zr, Ti, Sn,Nb, Y, Ga, and V (and optionally Si). In embodiments, (at least one of)the metal oxide precursor(s) in the further atomic layer depositionprocess is a Si metal oxide precursor. Especially, in furtherembodiments, in the further atomic layer deposition process the metaloxide precursor is selected from a group of metal oxide precursors ofmetals selected from a group consisting of Al, Hf, Ta, Zr, and Ti. Inembodiments, in the further atomic layer deposition process a metaloxide precursor is (also independently from the main ALD process)selected from a group consisting of Al(CH₃)₃, HAl(CH₃)₂, Hf(N(CH₃)₂)₄,Hf(N(CH₂CH₃)₂)₄, Hf[N(CH₃)(CH₂CH₃)]4, TaCl₅, Ta(N(CH₃)₂)₅,Ta{[N(CH₃)(CH₂CH₃)]3N(C(CH₃)₃)}, ZrCl₄, Zr(N(CH₃)₂)₄, TiCl₄, Ti(OCH₃)₄,and Ti(OCH₂CH₃)₄, and an oxygen source selected from a group consistingof H₂O and O₃ are applied. Additionally or alternatively, in the furtheratomic layer deposition process, a metal oxide precursor is selectedfrom a group consisting of Zn(C₂H₅)₂ and Zn(CH₃)₂. In yet furtherembodiments, additionally or alternatively, in the further atomic layerdeposition process the metal precursor is selected from a groupconsisting of (tert-butylimido)-tris (diethylamino)-niobium, NbF₅,NbCl₅, and Tris(ethylcyclopentadienyl)Yttrium.

Especially, in embodiments, in the main atomic layer deposition processand/or in the further atomic layer deposition process, a metal oxideprecursor is selected from a group of metal oxide precursors of metalsselected from a group consisting of Al, Zn, Hf, Ta, Zr, Ti, Si, Sn, Nb,Y, Ga, and V.

It turned out that deposition temperatures in the 200-350° C. range aremost suitable for alumina ALD on the primer layer (and the main sol-gelcoating layer), preferably the temperature is in the 250-300° C. range.Similar temperatures may be applied for ALD of other metal oxideprecursors for the ALD layer(s).

In specific embodiments, the main ALD coating layer comprises amultilayer with at least three layers having different chemicalcompositions and one or more of the layers comprise an oxide of Si(SiO₂). Especially, such SiO₂ layer is sandwiched between other layersof the multilayer. Hence, especially the (ALD) layer (of the multilayerof the main ALD coating layer) contacting the main sol-gel layer and therespective (ALD) layer contacting the primer layer does not consist ofSiO₂. Yet, in embodiments a further ALD coating layer contacting themain sol-gel coating layer may comprise SiO₂. Hence, in embodiments, inthe (main and/or further) atomic layer deposition process a metal oxideprecursor of Si is selected.

Especially, the main ALD alumina (or other metal oxide) layer has athickness of 3-250 nm, especially a thickness of as 5-250 nm, such as5-100 nm, even more especially a thickness of 5-50 nm, such asespecially 10-50 nm, even more especially a thickness of 20-50 nm.

Water gas penetration barrier properties of alumina ALD layers can befurther improved by depositing at least one additional layer of adifferent oxide material such as ZrO₂, TiO₂, Y₂O₃, Nb₂O₅, HfO₂, Ta₂O₅.Especially, the thickness of the additional material layer is in therange 1-40 nm, more preferably in the range 1-10 nm. Even moreespecially are nanolaminate stacks of alternating layers of Al₂O₃ and asecond oxide material from the group of ZrO₂, TiO₂, Y₂O₃, Nb₂O₅, HfO₂,SnO₂ Ta₂O₅. A suitable nanolaminate stack may be e.g. 20×(1 nm Al₂O₃ (10ALD cycles)+1 nm ZrO₂ (11 ALD cycles)) deposited at 250° C. to form a 40nm thick nanolaminated 2^(nd) coating on the primer layer (and/or themain sol-gel coating layer).

The invention especially provides in embodiments a method wherein themain ALD coating layer comprises a multilayer with layers havingdifferent chemical compositions, and wherein in the atomic layerdeposition process a metal oxide precursor is—amongst others—selectedfrom a group of metal oxide precursors of metals selected from a groupconsisting of Al, Zn, Hf, Ta, Zr, Ti, Sn, Y, Ga, Ge, V and Nb (andoptionally Si), especially the metal oxide precursor is selected from agroup of metal oxide precursors of metals selected from a groupconsisting of Al, Hf, Ta, Zr, and Ti. Also combinations of two or moreof such precursors may be used, e.g. a multilayer comprising alumina—amixed oxide of zirconium and hafnium-alumina, etc. In specificembodiments, in the main atomic layer deposition process, the metaloxide precursor for the two or more layers is selected from a group ofmetal oxide precursors of metals selected from a group consisting of Al,Hf, Ta, Zr, and Ti.

Hence, in embodiments the main ALD coating layer may comprise amultilayer with (n) layers having different chemical compositions, andwherein the multilayer comprises one or more layers comprising an oxideof one or more of Al, Zn, Hf, Ta, Zr, Ti, Sn, Y, Ga, Ge, V, and Nb (andoptionally Si), especially wherein the multilayer comprises one or morelayers comprising an oxide of one or more of Al, Hf, Ta, Zr, and Ti. Oneor more layers of such multilayers may also include mixed oxides, suchas indicated above.

In further specific embodiments, the method of the invention comprisessuccessively providing n layers (onto the primer layer by application ofthe main atomic layer deposition process), especially wherein each layerhas a layer coating layer thickness (d21) in the range of 1-50 nm,especially 1-20 nm, such as 1-15 nm. The layer coating thickness may inembodiments be at least 2 nm, such as at least 5 nm and e.g. be in therange of 5-40 nm, especially 5-25 nm. The number n of layers isespecially at least 2, such as at least 3, or at least 4. Inembodiments, n may be larger than 10. Yet, n is especially equal to orsmaller than 10, such as equal to or smaller than 5. In embodiments,2≤n≤10, or especially 2≤n≤5. It will be understood that an individuallayer may be provided by one or more ALD cycles. Further, especiallyadjacent (contacting) layers comprise different chemical compositions.In further embodiments, one or more layers comprise one or more metaloxides selected from a group of HfO₂, ZrO₂, TiO₂, Ta₂O₅, especiallywherein one or more (other) layers comprise Al₂O₃. It further appearedto be advantageous when the layer contacting the main sol-gel coatinglayer consist of HfO₂ and/or ZrO₂ and/or TiO₂ and/or Ta₂O₅. Hence, infurther embodiments, a layer contacting the main sol-gel coating layerconsist of one or more metal oxides selected from the group of HfO₂,ZrO₂, TiO₂, Ta₂O₅.

Hence, in specific embodiments, the method comprises successivelyproviding n (ALD) layers onto the primer layer by application of themain atomic layer deposition process (to provide the multilayer),wherein each layer has a layer coating layer thickness (d21) in therange of 1-50 nm, especially 1-20 nm, such as 1-15 nm, and wherein2<n≤50, especially 2<n≤20, such as 2≤n≤10, especially 2≤n≤5, wherein oneor more layers comprise one or more metal oxides selected from a groupof HfO₂, ZrO₂, TiO₂, Ta₂O₅, and wherein one or more layers compriseAl₂O₃, wherein a layer contacting the main sol-gel coating layer consistof one or more metal oxides selected from the group of HfO₂, ZrO₂, TiO₂,Ta₂O₅.

Especially the method is applied such that a (n ALD) multilayer coating(especially for the main ALD coating layer) is obtained including atleast two (ALD) layers (“AB”), even more especially at least threelayers (e.g. “ABA”), yet even more at least four layers. Yet moreespecially, at least a stack comprising two or more stack of subsets oftwo (ALD) layers (“AB”) is applied, such as (AB)_(n), wherein n is 2 ormore, such as 2-20, like 2-10.

Especially, at least one of the layers of the multilayer comprises anoxide of Al(optionally in combination with a further oxide of e.g. Si,or another metal oxide described herein), and at least one of the layersof the multilayer comprises one or more of an oxide of Hf, Zn, Ta, Zr,Ti, Y, Ga, Ge, V, Sn, and Nb. Such layer may optionally also include Al,Zn, Hf, Ta, Zr, Ti, Sn(Si,) Y, Ga, Ge, V, and Nb, wherein Al is in alayer together with one or more of the other indicated elements, whenthe other layer(s) of the multilayer comprise an oxide of alumina,respectively. The term “ALD multilayer” or “multilayer” as indicatedabove especially refers to layers having different chemicalcompositions. The phrase “layers having different chemical compositions”indicates that there are at least two layers having different chemicalcompositions, such as in the case of “ABC”, or in the case of (AB)_(n)(with n≥1).

Specific examples of (AB)₁ include multilayers wherein A is an oxide ofAl and wherein B is selected from one or more of an oxide of Al, Zn, Hf,Ta, Zr, Ti, Sn, Y, Ga, Ge, V, and Nb, wherein Al (and/or optionally Si)is in a layer together with one or more of the other indicated elements,when the other layer(s) of the multilayer comprise an oxide of alumina,respectively, especially wherein B is selected from one or more of anoxide of Hf, Zn, Ta, Zr, Ti, Y, Ga, Ge, V, and Nb, yet even moreespecially wherein B is selected from one or more of an oxide of Hf, Ta,Zr, and Ti, more especially wherein B is selected from one or more of anoxide of Hf, Ta, and Zr. In embodiments, B may further comprise an oxideof Si (optionally in combination with one or more of the oxides of Al,Zn, Hf, Ta, Zr, Ti, Sn, Y, Ga, Ge, V, and Nb). Especially, if (also) aSiO₂ layer is deposited with ALD, the SiO₂ (ALD) layer is deposited suchthat it not directly contacts the main sol-gel layer. For instance, theA layer (or B layer) of a multilayer may be an SiO₂ layer and the Blayer (or A layer) contacting the main sol-gel layer may be an (ALD)layer having another chemical composition.

This main ALD multilayer is thus especially provided on the primerlayer. The main sol-gel layer is especially provided on the main ALDmultilayer. Further, as indicated above, on top of the main sol-gellayer, optionally one or more further layers may be applied, especiallya further ALD layer may be provided on top of the main sol-gel layer.The further ALD layer may comprise an ALD multilayer, for instance anALD multilayer as described herein in relation to the main ALDmultilayer.

In further specific embodiments, the method further comprises (iv)providing a further ALD coating layer onto the luminescent core with themain sol-gel coating by application of a further atomic layer depositionprocess. Especially, thereby a further ALD coated luminescent particleis provided. In the further atomic layer deposition process especially ametal oxide precursor is selected from a group of metal oxide precursorscomprising Al, Zn, Hf, Ta, Zr, Ti, Sn, Nb, Y, Ga, and V (and optionallySi), especially comprising Al, Hf, Ta, Zr, Ti. In embodiments, thefurther ALD coating layer has a further ALD coating layer thickness (d4)in the range of 2-50 nm, especially 10-50 nm, such as 10-20 nm. Thefurther ALD coating layer especially has a chemical compositiondiffering from the chemical composition of the main sol-gel coatinglayer.

In further embodiments, the further ALD coating layer (optionally)comprises (is provided comprising) a further multilayer with two or more(further sub) layers having different chemical compositions, wherein oneor more of the layers comprise metal oxides selected from a group ofAl₂O₃, TiO₂, ZrO₂, HfO₂, SnO₂, ZnO and Ta₂O₅, and wherein the two ormore layers have a chemical composition differing from the chemicalcomposition of the main sol-gel coating layer. In specific embodiments,in the further atomic layer deposition process, the metal oxideprecursor for the two or more (further sub) layers (of the furthermultilayer) is selected from a group of metal oxide precursors of metalsselected from a group consisting of Al, Hf, Ta, Zr, and Ti. The metaloxide precursor for the two or more (further sub) layers (of the furthermultilayer) is especially selected from a group of metal oxideprecursors comprising Al, Hf, Ta, Zr, Ti.

Hence, in specific embodiments the main ALD coating layer comprises amultilayer with a stack of layers, with adjacent layers having differentchemical compositions. Especially, the layers of the multilayer haveeach independently thicknesses in the range of 1-40 nm, especially 1-10nm. Further, especially, the multilayer comprises one or more aluminalayers and one or more metal oxide layers, with the metal selected froma group of Hf, Ta, Zr and Ti.

Therefore, in specific embodiments in the atomic layer depositionprocess a metal oxide precursor selected from a group consisting ofAl(CH₃)₃, HAl(CH₃)₂, Hf(N(CH₃)₂)₄, Hf(N(CH₂CH₃)₂)₄, Hf[N(CH₃)(CH₂CH₃)]4,TaCl₅, Ta(N(CH₃)₂)₅, Ta{[N(CH₃)(CH₂CH₃)]3N(C(CH₃)₃)}, ZrCl₄,Zr(N(CH₃)₂)₄, TiCl₄, Ti(OCH₃)₄, and Ti(OCH₂CH₃)₄, and an oxygen sourceselected from a group consisting of H₂O and 03 are applied. As indicatedabove, also two or more different metal oxide precursors and/or two ormore different oxygen sources may be applied.

Further, in embodiments of the method in the main atomic layerdeposition process and/or the further atomic layer deposition process,especially in the main atomic layer deposition process, a multilayer isprovided, with layers having different chemical compositions, whereinone or more layers comprise tantalum oxide (especially Ta₂O₅). Hence,the invention also provides in embodiments luminescent material, whereinthe main ALD coating layer comprises a multilayer with layers havingdifferent chemical compositions, wherein one or more layers mayespecially comprise Ta₂O₅. Further, in embodiments of the method in the(main) atomic layer deposition process, a multilayer is provided, withlayers having different chemical compositions, wherein one or morelayers comprise one or more of tantalum oxide (especially Ta₂O₅),hafnium oxide, titanium oxide, and zirconium oxide. Hence, the inventionalso provides in embodiments luminescent material, especiallyluminescent particles, wherein the main ALD coating layer comprises amultilayer with layers having different chemical compositions, whereinone or more layers may especially comprise one or more of tantalumoxide, hafnium oxide, titanium oxide and zirconium oxide. For instance,the multilayer stack may also include a stack with alternating layerswherein e.g. alumina alternates with one or more of tantalum oxide(especially Ta₂O₅), hafnium oxide, titanium oxide, and zirconium oxide,such as a stack comprising e.g. alumina-tantalumoxide-alumina-Hafnia-alumina-tantalum oxide, or alumina-titaniumoxide-alumina, etc.

Further, it appeared that when first an ALD coating was provided on corewithout a primer layer the ALD layer was less uniform than desirable. Toobtain a good ALD layer directly on the core surface, the ALD layerthickness may in such embodiments have to be increased more than inprinciple would be necessary, which may lead to an unnecessary reductionin transmission (even though in some cases small). Further, it appearedthat after providing the primer layer at the core (even when not beingcompletely conformal), an ALD coating coats more easily to the core.

As indicated above, the main sol-gel layer that typically has an averagethickness in the range of 50-700 nm, such as 50-600 nm, especially75-500 nm, such as especially 100-500 nm, and is formed by a sol-geltype process. In such process, an inorganic network is formed from ahomogeneous solution of precursors by subsequent hydrolysis to form asol (colloidal suspension) and condensation to then form a gel(cross-linked solid network) that is chemically bonded to the powdersurfaces. Preferably, the (main) sol-gel coating layer material issilica and the sol-gel deposition method corresponds to the so-calledStöber reaction as described in Stöber, W., A. Fink, et al. “Controlledgrowth of monodisperse silica spheres in the micron size range.” Journalof Colloid and Interface Science 26(1): 62-69. To this end the (coatedor uncoated) luminescent particle is dispersed in an alcohol such as analiphatic alcohol R—OH such as methanol CH₃OH, ethanol C₂H₅OH oriso-propanol C₃H₇OH followed by addition of ammonia (NH₃ solution inwater) and a silicon alkoxide precursor. The silicon alkoxide precursordissolves in the alcohol+ammonia mixture and starts to hydrolyze. Aconformal silica coating is formed on top of the (coated or uncoated)particle surfaces by reaction of the hydrolyzed, yet dissolved solspecies with reactive groups of the particle surfaces (e.g. amine orsilanol groups) followed by a seeded growth process that consists ofhydrolysis, nucleation and condensation reactions steps.

The term “(coated or uncoated) particle surface” in relation to thesol-gel coating process may especially relate to the surface of theparticle (luminescent core) and/or the surface of the washing resultlayer (especially the oxide-containing layer) on the particle(luminescent core), especially in relation to the primary sol-gelcoating process. The term may further relate to the surface of the mainALD coating, especially in relation to the main sol-gel coating process.

The silicon alkoxide recursor is especially selected from a group ofcompounds that is formed by

wherein a) R1, R2, R3 are hydrolysable alkoxy groups and R4 is selectedfrom a group of C1-C6 linear alkyl groups, hydrolysable alkoxy groupsand a phenyl group, or b) R1, R2, R3 are individually selected from—OCH₃ and —OC₂H₅ and R4 is selected from —CH₃, —C₂H₅, —OCH₃, —OC₂H₅ anda phenyl group. Optionally, the silicon based polymer is obtained from amaterial from the group of:

Hence, in embodiments of the method, in the main sol-gel coatingprocess, a silicon alkoxide precursor is used, wherein the siliconalkoxide precursor is especially selected from a group of compoundsconsisting of

wherein a) R1, R2, R3 are hydrolysable alkoxy groups and R4 is selectedfrom a group of C1-C6 linear alkyl groups, hydrolysable alkoxy groupsand a phenyl group, or b) R1, R2, R3 are individually selected from—OCH₃ and —OC₂H₅ and R4 is selected from —CH₃, —C₂H₅, —OCH₃, —OC₂H₅ anda phenyl group.

In further embodiments of the method, in the main sol-gel coatingprocess, a silicon alkoxide precursor is used, and the silicon alkoxideprecursor is selected from a group consisting of

In further embodiments of the method, in the primary sol-gel coatingprocess, a silicon alkoxide precursor is used, and especially thesilicon alkoxide precursors may be a silicon alkoxide precursor asdescribed herein in relation to the main sol-gel coating process. Thesilicon alkoxide precursor in the primary sol-gel coating process may beindependently selected from the silicon alkoxide precursor in the mainsol-gel coating process.

Especially, the silicon alkoxide precursor (in the main and/or primarysol-gel coating process) is selected from a group of Si(OCH₃)₄ orSi(OC₂H₅)₄, more especially Si(OC₂H₅)₄ is used as silicon alkoxideprecursor. Similar precursors but based on another metal such as e.g. Almay also be used.

A typical sol-gel coating process may comprise the following stages: (a)particles or powder, especially luminescent cores) (optionally with theoxide-containing layer and/or the main ALD coating layer) are suspendedin an alcohol-aqueous ammonia solution mixture while stirring orsonication. To improve particle dispersion, the particles (cores/powder)can also first be mixed with alcohol and a small amount of a silicon (orother metal) alkoxide before the ammonia solution is added. (b) Asilicon (or other metal) alkoxide precursor is added under agitation ofthe suspension. Typical concentrations of silicon (or other metal)alkoxide, ammonia and water in the alcohol solvent are 0.02-0.7,0.3-1.5, and 1-16 mole/l, respectively. (c) The suspension is stirred orsonicated until the coating has formed. (d) The coated powder is washedwith alcohol and dried followed by calcination in air or vacuum at200-300° C.

Hence, in embodiments the main sol-gel coating process comprises: (iiia)providing a mixture of an alcohol, ammonia, water, the luminescentcore(s) with the (primer layer and) the main ALD coating layer and ametal alkoxide precursor while agitating the mixture, and allowing amain sol-gel coating layer to be formed on the main ALD coating layer,wherein the metal alkoxide precursor is especially titanium alkoxide,silicon alkoxide, or aluminum alkoxide; and (iiib) retrieving theluminescent core(s) with (the primer layer,) the main ALD coating layerand the main sol-gel coating layer from the mixture and optionallysubjecting the retrieved luminescent core(s) with the primer layer, themain ALD coating layer and the main sol-gel coating layer to a heattreatment to provide the luminescent particle(s) with hybrid coating.

Hence, in further embodiments, the primary sol-gel coating processcomprises: (ib1) providing a mixture of an alcohol, ammonia, water, theluminescent core(s) optionally with the washing result layer, especiallythe luminescent core(s) and the washing result layer (or the luminescentcore(s) comprising the washing result layer) and a metal alkoxideprecursor while agitating the mixture, and allowing the primary sol-gelcoating layer to be formed on the washing result layer and/or on theluminescent core(s) without a washing result layer, especially on thewashing result layer, wherein the metal alkoxide precursor is especiallyselected from titanium alkoxide, silicon alkoxide, or aluminum alkoxide;and (ib2) retrieving the luminescent core(s) with the (washing resultlayer and the) primary sol-gel coating layer from the mixture andoptionally subjecting the retrieved luminescent core(s) with (thewashing result layer and) the primary sol-gel coating layer to a heattreatment to provide the luminescent core(s) comprising (with) theprimer layer on the luminescent core.

The process of retrieving the core(s) (with the respective (coating)layers) from the mixture may e.g. include one or more of filtration,centrifuging, decanting (the liquid over a precipitate), etc. The heattreatment may include one or more of drying and calcination, especiallyboth, i.e. e.g. a drying stage at a temperature in the range of 70-130°C. followed by a calcination stage (in air; or vacuum or an (other)inert atmosphere). Hence, during part of the time of the heat treatment,the (coated) luminescent may be in an inert environment, such as vacuum,or one or more of N₂ and a noble gas, etc. The heat treatment seems toimprove the stability of the luminescent material. Further, as indicatedabove, in the (main and/or primary) sol-gel coating process a silicon(or other metal; though the formula below refers to Si) alkoxideespecially a precursor may be used selected from a group of compoundsconsisting of:

wherein R1, R2, R3 are selected from a group consisting of hydrolysablealkoxy moieties and R4 is selected from a group consisting of C1-C6linear alkyl moieties, hydrolysable alkoxy moieties, and a phenylmoiety. Optionally other ligands than alkoxides may be applied inprecursor for the sol-gel process.

The particles obtained with a sol-gel coating process may optionallyinclude more than one nucleus. For instance in the case of quantum dots,agglomerates with a (primary and/or main) sol-gel coating may beobtained. Hence, the silica precursor (or other metal oxide precursor)can also coat multiple QDs (especially comprising the main ALD coatinglayer) with thin single shells to form a coated agglomerate. This mayamongst others depend upon the concentration of the quantum dots, etc.

Above, the precursors for the sol-gel coating are especially describedin relation to a silicon alkoxide precursor. However, also aluminum (oranother metal) alkoxide precursor(s) may be applied. Further, also acombination of two or more chemically different precursors may beapplied for providing the sol-gel coating layer or first coating layer.

The term “sol-gel coating process” may also relate to a plurality ofsol-gel coating processes. With a plurality of sol-gel coatingprocesses, especially a plurality of main sol-gel coating processes, onemay provide a (multi)layer substantially comprising the same compositionthrough the entire layer thickness (when e.g. in the (main) sol-gelcoating process each coating stage or step includes depositingsubstantially the same material), or may provide a multilayer with twoor more layers having different compositions, such as a stack of two ormore (sol-gel) layers with two or more different compositions,respectively. An example may e.g. be a SiO₂—Al₂O₃ (sol-gel) multilayer,such as a stack of three or more (sol-gel) layers wherein SiO₂ and Al₂O₃alternate (see also above).

To facilitate the deposition of the main ALD coating layer, theluminescent core may comprise the primer layer. In embodiments, theprimer layer comprises the primary sol-gel coating layer (optionallyincluding a multilayer), especially provided by the (primary) sol-gelcoating process. The primary sol-gel coating layer may facilitate thedeposition of a (thin) conformal main ALD coating layer. To furthersupport the deposition of the main ALD coating layer, the surface of thecore is in embodiments cleaned before providing the primary sol-gellayer and/or the main ALD coating layer.

To facilitate the main ALD deposition process, and especially to enablethe deposition of a single ALD layer or multiple layers with as fewdefects as possible, any chemically reactive contamination (also knownas “second phases”) that may be present in the powder (raw product,especially comprising a plurality of luminescent cores) may be removed.Preferably, small particles, typically having a sub-micron dimension,and that may stick to the surface of the phosphor particle (luminescentcore) are removed as well before depositing the main ALD coatingprocess. The cleaning of the surface of the core may especially comprisea (chemical) washing process. In embodiments, the core may be washed byapplying a washing process, by applying an aqueous liquid. Such aqueousliquid may comprise an acid or a base or may e.g. consist of water (witha neutral pH). Water may e.g. be applied for removing small unwantedparticles and part of the second phases. Yet, for removing additionalimpurities the pH of the aqueous liquid may be changed, e.g. topH-values of at least 8, especially at least 9, or to pH values below 6,especially below 5. This way, e.g. impurities may be dissolved. Infurther embodiments, a non-aqueous solvent may be applied, especiallyfor particles (cores) that may be sensitive to water. Hence, especiallyan aqueous liquid comprising additives (e.g. to change the pH), anon-aqueous solvent, or a combination of these liquids/solvents may beapplied. Therefore, the cleaning/washing process may be indicated as a“chemical washing process”, especially by applying a washing solvent(including an aqueous liquid).

If the chemical stability of the phosphor (luminescent core) in water,bases, and/or acids is limited, washing procedures with very mildconditions may be applied to remove 2^(nd) phases without dissolving(part of) the phosphor particles. This can be achieved by choosing weakinstead of strong acids with pK_(a) values that may be selecteddepending on the stability of the phosphor and impurity phases (seebelow). Additionally or alternatively degradation of the phosphor in thewashing process may be avoided by first applying a non-aqueous solventto the luminescent core(s) (phosphors) to provide a suspension (of theluminescent particles/cores) and successively add a weak acid (or base)in such a way that the total amount of water and the acid concentrationare sufficient to only dissolve the impurity phases.

A washing solvent having a pH less than 7 is useful for hydrolysissensitive luminescent materials, as such luminescent materials asdisclosed herein react as a base in aqueous media. The acidity of suchwashing solvent may be low. Specifically, organic acids such as aceticacids diluted in a polar solvent with a low proton concentration, suchas aliphatic alcohols, e.g. ethanol or isopropanol, as disclosed above,may be used as the washing solvent. The more sensitive the luminescentmaterial is, the more diluted (i.e., the lower the proton concentration)the washing solvent should be, as the goal of the washing process is toremove basic impurity phases and create a surface at the particulateluminescent material that aids adhesion of the subsequent primer layerwithout degrading the luminescent material. In general, a washingsolvent of Thus, if a person having ordinary skill in the art observesthat too much of the luminescent particle material is degrading ordissolving in the washing process, a reduction of the acidconcentration, a replacement of the solvent by another solvent with alower dielectric contant and/or a cooling of the washing suspension mayreduce the amount of degradation.

After washing, the number of fine particles in the phosphor powder maybe further reduced by sedimentation in non-reactive liquids (typicallypolar organic solvents, like e.g. water-free ethanol, or otheralcohols). In embodiments, ultrasound is applied to the suspensionbefore sedimentation to better detach and disperse fine particles fromlarger phosphor grains.

As a result of the chemical washing process a thin layer may be formedat the particle (core) surface with a different composition compared tothe nominal composition of the luminescent particle (luminescent core).That is, the surface composition of the particulate luminescent particlemay differ somewhat from the overall composition of the particulateluminescent material. Especially the thin layer (i.e., surface) maycomprise a higher oxygen concentration (content) than the (nominalcomposition of) the luminescent core. Applying the chemical washing mayin embodiments provide a washing result layer onto the luminescent core.The washing process may provide a surface (washing result layer) thataids in the adhesion of the subsequent primer layer. The thinlayer/surface may contain, for example, alkaline earth elements (e.g.,strontium), aluminum, and oxide when, for example alkaline earthaluminate type hydrolysis sensitive luminescent materials such as thosedisclosed herein are to be coated. The surface layer may containelements such as lithium, silicon, europium, carbon and/or hydrogen,depending on the luminescent material. The washing result layerespecially comprises an oxide-containing layer. The washing result layernot necessarily is conformally and/or entirely covering the surface ofthe luminescent core (see also above in relation to the primer layer).The washing result layer may not be a continuous layer and may e.g.comprise a plurality of layer section, each covering only a section ofthe surface of the luminescent core. The washing result layer may inembodiments be evenly distributed over (the surface of) the luminescentcore. The washing result layer may in embodiments cover the core for atleast 30%, such as at least 50%, especially at least 75%, such as atleast 90%, or especially at least 95% or even more especially at least99%, of the surface of the core.

Experimentally it was noticed that even by applying a mild washingsolvent the washing result layer is observable. Especially, in the caseof nitride or oxonitride compounds, the O:N ratio may be higher in thewashing result layer. The luminescent core especially comprises nitrideor oxonitride compounds. In embodiments, the luminescent material,especially of the luminescent core, comprises a nitride luminescentmaterial (core). In further embodiments, the luminescent material,especially of the luminescent core, (also) comprises an oxonitrideluminescent material (core).

Hence, in further embodiments, the method further comprises (ia)providing a washing result layer onto the luminescent core byapplication of a chemical washing process, especially wherein thewashing result layer comprises an oxide-containing layer. In furtherembodiments, the application of the chemical washing process includesdrying the luminescent core(s) after removing the washing solvent. Thewashing result layer may be provided during the washing of theluminescent core with the washing solvent and/or during drying of theluminescent core. In further embodiments, the primary sol-gel coating isprovided after application of the chemical washing process (optionallyincluding the drying of the luminescent core). Application of thechemical washing process may provide a washed luminescent particlecomprising the washing result layer onto the luminescent core.

Hence, in further embodiments, the method further comprises providing aprimary sol-gel coating layer onto luminescent core and the washingresult layer (or onto the luminescent core comprising the washing resultlayer) by application of a primary sol-gel coating process, therebyproviding the primer layer comprising the washing result layer and theprimary sol-gel layer, and especially wherein the primer layer has aprimer layer thickness (d1) in the range of 0.1-5 nm.

The washing solvent may in embodiments comprise an aqueous solvent.Herein, this (washing with a solvent comprising an aqueous solvent) mayalso be indicated as a “wet chemical washing process”. In embodiments,e.g. the washing solvent comprises a strong acid or a strong base. Theseterms are known in the art. Examples of strong acids are HCl, HBr,HClO₄, HI, HNO₃. Examples of strong bases are e.g. NaOH, KOH, CaOH. Yet,in further embodiments, the washing solvent comprises a weak acid or aweak base. Examples of weak acids that may be used are e.g. acetic acid,formic acid, hydrofluoric acid, trichloroacetic acid, citric acid,oxalic acid etc. Examples of weak bases are e.g. ammonia, sodiumbicarbonate, alanine, and methylamine.

The weak acid or the weak base may especially be selected having apK_(a) value or a pK_(b) value respectively higher than 3, especiallyequal to or higher than 4 (in water at room temperature). Inembodiments, the washing solvent comprises one or more weak acidsselected from a group of acetic acid, formic acid, hydrofluoric acid,trichloroacetic acid, citric acid, oxalic acid. The washing solvent mayespecially comprise formic acid or acetic acid. In further embodiments,the washing solvent comprises one or more weak based ammonia, sodiumbicarbonate, alanine, and methylamine. The washing solvent mayespecially comprise a combination of a non-aqueous fluid and a weak acidor a weak base. For instance, the washing solvent may in embodimentscomprise an alcohol (e.g. propanol, isopropyl alcohol, ethanol, (cyclo)hexanol or any other alcohol with one or more hydroxy groups) and a weakacid, e.g. formic acid and/or acetic acid. Alternatively the washingsolvent may in embodiments comprise mixtures of an alcohol and a polyol.In embodiments, e.g. the washing solvent comprises a mixture of ethanoland as triethylene glycol and especially traces as water acting as adissolution catalyst. Such combination may advantageously be applied forwashing luminescent particles (cores) that may easily degrade under theinfluence of water. The washing solvent may in embodiments comprise lessthan 50 wt % water (relative to the weight of the washing solvent). Thewashing solvent may e.g. comprise equal to or less than 40 wt % water,such as equal to or less than 35 wt % water. In embodiments, the washingsolvent may comprise no more than 25 wt % water. Especially, the washingsolvent (comprising a (weak) base or (weak) acid) comprises at least 5wt % water, such as at least 10 wt % water. Yet in embodiments, thewashing solvent is a non-aqueous washing solvent. Further, theapplication of weak acids (and weak bases) may have a further benefit inthat they may provide a pH buffering function. As such, the pH of thewashing solvent may not substantially change if an extra amount of theweak acid (or base) is added to the washing solvent (e.g. if not allimpurities are removed by the washing solvent yet). Using weak acids orweak bases may increase the robustness of the wet washing process.

Hence, in further embodiments, the chemical washing process mayespecially comprise a wet chemical washing process comprising (i)washing the luminescent core by applying a washing solvent (process),wherein the washing solvent comprises an (weak) acid or a (weak) base,especially a (weak) acid, and wherein the washing solvent comprisesequal to or less than 50% wt/wt water, especially in the range of 10-35%wt/wt water and optionally (ii) successively subjecting the luminescentcore to a drying treatment, thereby providing the luminescent corecomprising the washing result layer on(to) the luminescent core.

Especially based on the washing process (optionally including the dryingtreatment), the oxygen-containing layer may be provided on theluminescent particle.

The different (coating) layers that may be configured at the luminescentcore (the primary layer, the main ALD-layer, the main sol-gel coatinglayer and the further ALD coating layer) are especially lighttransmitting. This means that at least a portion of the light, whichimpinges on the respective layers, is transmitted through the respectivelayer. Thus, the different (coating) layers may be fully or partiallytransparent or may be translucent. In an embodiment, more than 90% ofthe (visible) light which impinges on the (coating) layers istransmitted through the (coating) layers. The (coating) layers may belight transmitting because of characteristics of the materials of whichthe coating layers are made. For example, the coating layer may be madefrom a material which is transparent, even if the layer is relativelythick. In another embodiment, one or more of the (coating) layers arethin enough such that the respective layer becomes light transmittingwhile the material of which the layer is manufactured is not transparentor translucent when manufactured in relatively thick layers. Thematerials described herein are all transmissive for (visible) light orcan be made in suitable layer thicknesses that are transmissive for(visible) light.

In a further aspect, the invention also provides a lighting devicecomprising a light source configured to generate light source radiation,especially one or more of blue and UV, and a wavelength convertercomprising the luminescent material as described herein, wherein thewavelength converter is configured to convert at least part of the lightsource radiation into wavelength converter light (such as one or more ofgreen, yellow, orange and red light). The wavelength converter isespecially radiationally coupled to the light source. The term“radiationally coupled” especially means that the light source and theluminescent material are associated with each other so that at leastpart of the radiation emitted by the light source is received by theluminescent material (and at least partly converted into luminescence).Hence, the luminescent cores of the particles can be excited by thelight source radiation providing luminescence of the luminescentmaterial in the core. In embodiments, the wavelength converter comprisesa matrix (material) comprising the luminescent material (particles). Forinstance, the matrix (material) may comprise one or more materialsselected from a group consisting of a transmissive organic materialsupport, such as selected from a group consisting of PE (polyethylene),PP (polypropylene), PEN (polyethylene napthalate), PC (polycarbonate),polymethylacrylate (PMA), polymethylmethacrylate (PMMA) (Plexiglas orPerspex), cellulose acetate butyrate (CAB), silicone, polyvinylchloride(PVC), polyethylene terephthalate (PET), (PETG) (glycol modifiedpolyethylene terephthalate), PDMS (polydimethylsiloxane), and COC (cycloolefin copolymer). Alternatively or additionally, the matrix (material)may comprise an epoxy resin.

When forming such a lighting device, use of the coating disclosed hereinallows hydrolysis sensitive phosphors to be used as the luminescentmaterial in the wavelength converter. In particular, luminescentmaterial that would likely degrade under the conditions used to form thematrix in which the luminescent particles are embedded to form thewavelength converter may be used in lighting devices (as shown in FIG. 3below). For example, alkaline earth aluminate luminescent materials, orluminescent materials that have alkaline earth aluminate type surfacelayers after the washing process disclosed herein. Such hydrolysissensitive luminescent materials include, for example, luminescentparticles disclosed above that include a luminescent material selectedfrom (the) SrLiAl₃N₄:Eu²⁺ (class), in which, optionally also part of Srmay be replaced by another alkaline earth metal (group 2 elements of theperiodic table). And also, for example, luminescent material (orphosphor) selected from a group consisting of (Sr,Ca)LiAl₃N₄:Eu,(Sr,Ca,Ba)Li_(d)Mg_(a)Al_(b)N₄:Eu, with 0≤a≤4; 0≤b≤4; 0≤d≤4; and a+b+d=4and 2a+3b+d=10, and (Sr,Ba)Li₂Al_(2-z)Si_(z)O_(2-z)N_(2+z):Eu, wherein0≤z≤0.1 disclosed above. And also, for exampleSrLi₂Al_(2-x)SiO_(2-x)N_(2+x):Eu or (Sr,Ca)SiAlN₃:Eu disclosed above.Use of the coating disclosed herein allows such hydrolysis sensitiveluminescent materials to be used in processes for forming wavelengthconverters, for example, processes for forming wavelength converters inwhich luminescent particles are embedded within a matrix, such assilicone resins, which otherwise may degrade the uncoated luminescentmaterials.

The lighting device may be part of or may be applied in e.g. officelighting systems, household application systems, shop lighting systems,home lighting systems, accent lighting systems, spot lighting systems,theater lighting systems, fiber-optics application systems, projectionsystems, self-lit display systems, pixelated display systems, segmenteddisplay systems, warning sign systems, medical lighting applicationsystems, indicator sign systems, decorative lighting systems, portablesystems, automotive applications, green house lighting systems,horticulture lighting, or LCD backlighting.

As indicated above, the lighting unit may be used as backlighting unitin an LCD display device. Hence, the invention also provides an LCDdisplay device comprising the lighting unit as defined herein,configured as backlighting unit. The invention also provides in afurther aspect a liquid crystal display device comprising a backlighting unit, wherein the back lighting unit comprises one or morelighting devices as defined herein.

Especially, the light source is a light source that during operationemits (light source radiation) at least light at a wavelength in therange of 200-490 nm, especially a light source that during operationemits at least light at wavelength in the range of 400-490 nm, even moreespecially in the range of 440-490 nm. This light may partially be usedby the wavelength converter nanoparticles (see further also below).Hence, in a specific embodiment, the light source is configured togenerate blue light. In a specific embodiment, the light sourcecomprises a solid state LED light source (such as a LED or laser diode).The term “light source” may also relate to a plurality of light sources,such as 2-20 (solid state) LED light sources. Hence, the term LED mayalso refer to a plurality of LEDs. The term white light herein, is knownto the person skilled in the art. It especially relates to light havinga correlated color temperature (CCT) between about 2000 and 20000 K,especially 2700-20000 K, for general lighting especially in the range ofabout 2700 K and 6500 K, and for backlighting purposes especially in therange of about 7000 K and 20000 K, and especially within about 15 SDCM(standard deviation of color matching) from the BBL (black body locus),especially within about 10 SDCM from the BBL, even more especiallywithin about 5 SDCM from the BBL. In an embodiment, the light source mayalso provide light source radiation having a correlated colortemperature (CCT) between about 5000 and 20000 K, e.g. direct phosphorconverted LEDs (blue light emitting diode with thin layer of phosphorfor e.g. obtaining of 10000 K). Hence, in a specific embodiment thelight source is configured to provide light source radiation with acorrelated color temperature in the range of 5000-20000 K, even moreespecially from the range of 6000-20000 K, such as 8000-20000 K. Anadvantage of the relative high color temperature may be that there maybe a relatively high blue component in the light source radiation.

The term “controlling” and similar terms especially refer at least todetermining the behavior or supervising the running of an element.Hence, herein “controlling” and similar terms may e.g. refer to imposingbehavior to the element (determining the behavior or supervising therunning of an element), etc., such as e.g. measuring, displaying,actuating, opening, shifting, changing temperature, etc. Beyond that,the term “controlling” and similar terms may additionally includemonitoring. Hence, the term “controlling” and similar terms may includeimposing behavior on an element and also imposing behavior on an elementand monitoring the element. The controlling of the element can be donewith a control system, which may also be indicated as “controller”. Thecontrol system and the element may thus at least temporarily, orpermanently, functionally be coupled. The element may comprise thecontrol system. In embodiments, the control system and element may notbe physically coupled. Control can be done via wired and/or wirelesscontrol. The term “control system” may also refer to a plurality ofdifferent control systems, which especially are functionally coupled,and of which e.g. one control system may be a master control system andone or more others may be slave control systems. A control system maycomprise or may be functionally coupled to a user interface.

The control system may also be configured to receive and executeinstructions form a remote control. In embodiments, the control systemmay be controlled via an App on a device, such as a portable device,like a Smartphone or I-phone, a tablet, etc. The device is thus notnecessarily coupled to the lighting system but may be (temporarily)functionally coupled to the lighting system.

Hence, in embodiments the control system may (also) be configured to becontrolled by an App on a remote device. In such embodiments the controlsystem of the lighting system may be a slave control system or controlin a slave mode. For instance, the lighting system may be identifiablewith a code, especially a unique code for the respective lightingsystem. The control system of the lighting system may be configured tobe controlled by an external control system which has access to thelighting system on the basis of knowledge (input by a user interface ofwith an optical sensor (e.g. QR code reader) of the (unique) code. Thelighting system may also comprise means for communicating with othersystems or devices, such as on the basis of Bluetooth, WIFI, LiFi,ZigBee, BLE or WiMAX, or another wireless technology.

The system, or apparatus, or device may execute an action in a “mode” or“operation mode” or “mode of operation”. Likewise, in a method an actionor stage, or step may be executed in a “mode” or “operation mode” or“mode of operation” or “operational mode”. The term “mode” may also beindicated as “controlling mode”. This does not exclude that the system,or apparatus, or device may also be adapted for providing anothercontrolling mode, or a plurality of other controlling modes. Likewise,this may not exclude that before executing the mode and/or afterexecuting the mode one or more other modes may be executed.

However, in embodiments a control system may be available, that isadapted to provide at least the controlling mode. Would other modes beavailable, the choice of such modes may especially be executed via auser interface, though other options, like executing a mode independence of a sensor signal or a (time) scheme, may also be possible.The operation mode may in embodiments also refer to a system, orapparatus, or device, that can only operate in a single operation mode(i.e. “on”, without further tunability).

Hence, in embodiments, the control system may control in dependence ofone or more of an input signal of a user interface, a sensor signal (ofa sensor), and a timer. The term “timer” may refer to a clock and/or apredetermined time scheme.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described, by way of exampleonly, with reference to the accompanying schematic drawings in whichcorresponding reference symbols indicate corresponding parts, and inwhich:

FIG. 1 schematically depicts aspects of a luminescent particle;

FIG. 2 a-2 b schematically depict some further aspects of a luminescentparticle;

FIG. 3 schematically depicts a lighting device;

FIG. 4 a-4 b show a SEM and a TEM image of a luminescent particle;

FIGS. 5 a-5 b show some experimental results wherein embodiments of theinvention are compared to prior art luminescent materials.

FIGS. 6 a-6 b show, respectively, cross-sectional and top schematicviews of an array of pcLEDs.

FIG. 7 a shows a schematic top view of an electronics board on which anarray of pcLEDs may be mounted, and FIG. 7 b similarly shows an array ofpcLEDs mounted on the electronic board of FIG. 7 a.

FIG. 8 a shows a schematic cross-sectional view of an array of pcLEDsarranged with respect to waveguides and a projection lens. FIG. 8 bshows an arrangement similar to that of FIG. 8 a , without thewaveguides.

FIG. 9 schematically illustrates an example camera flash systemcomprising an adaptive illumination system.

FIG. 10 schematically illustrates an example display (e.g., AR/VR/MR)system that includes an adaptive illumination system.

The schematic drawings are not necessarily to scale.

DETAILED DESCRIPTION OF THE EMBODIMENTS

FIG. 1 schematically depicts an embodiment of the luminescent particles100. The luminescent particle 100 comprises a luminescent core 102comprising a primer layer 105 on the luminescent core 102. Herein theluminescent core 102 with the primer layer 105 is also referred to as aprimer layer 105 comprising luminescent particle 100. The primer layer105 has a chemical composition differing from the chemical compositionof the core 102. The luminescent core 102 may include e.g. micrometerdimensional particles of a luminescent nitride or sulfide phosphor butmay also include other (smaller) material such as luminescentnanoparticles (see further FIG. 2 b ).

The luminescent particle 100 further comprises a main ALD coating layer120. In the depicted embodiment the main ALD coating layer 120 comprisesa multilayer 1120 with three layers 1121, layer 1121 a, layer 1121 b,and layer 1121 c. The three layers 1121 a, 1121 b, 1121 c especiallyhave (at least two) different chemical compositions. Especiallyadjacently (and contacting) arranged layers 1121 have differentcompositions. Moreover, one or more of the layers 1121 of the multilayer1120 may have chemical compositions (also) differing from the chemicalcomposition of the primer layer 105. The layers 1121 may in embodimentse.g. comprise different oxides of Al, Zn, Hf, Ta, Zr, Ti, Sn, Nb, Y, Ga,and V. Additionally or alternatively, the layers 1121 may comprise Siand/or Ge. Especially one of the layers 1121 may be an alumina layer.

The luminescent particle 100 further comprises a main sol-gel coatinglayer 130, especially having a chemical composition differing from oneor more of the layers 1121 of the multilayer 1120. The figure furthershows that main ALD coating layer 120 is arranged between the primerlayer 105 and the main sol-gel layer 130. Especially, adjacentlyarranged/contacting coating layers may have different compositions. Inthe depicted figure, layer 1121 a especially has a composition thatdiffers from the composition of the main sol-gel layer 130. Layer 1121 cespecially has a composition that differs from the composition of theprimer layer 105. The hybrid coating of the embodiment in FIG. 1 thuscomprises a primer layer 105, a main ALD layer 120 and a main sol-gelcoating layer 130. In further embodiments, see e.g. FIG. 2 a , thehybrid coating further comprises a further ALD coating layer 140.

The embodiment of FIG. 2 a also comprises a further ALD coating layer140 arranged on the main sol-gel coating layer 130. In the depictedembodiment, the further ALD coating layer 140 (also) comprises a furthermultilayer 1140 comprising two (further sub) layers 1141, 1141 a, 1141 b(of the further multilayer 1140). Yet, in other embodiments the furtherALD coating layer 140 is (deposited as) a single layer. In FIG. 2 a alsothe thicknesses of the layers are indicated. It is noted that thethicknesses are not to scale and only are depicted to explain themeaning of the terms and show the location. The primer layer thicknessis indicated by the reference d1. The primer layer thickness d1 may bein the range of 0.1-5 nm. The ALD coating layer thickness is indicatedwith the reference d2. The ALD coating layer thickness d2 may especiallybe in the range of 5-250 nm. The thickness of the main sol-gel coating130 is indicated with reference d3. The main sol-gel coating layerthickness d3 is generally larger than the ALD coating layer thicknessd2. The main sol-gel coating layer thickness d3 is especially in therange of 50-700 nm. The depicted embodiment comprises a multilayer 1120with three layers 1121, each layer 1121 having a layer coating layerthickness d21 in the range of 1-20 nm. In the depicted embodiment, thelayer coating thickness d21 of the three layers 1121 is about the same.The layer coating thickness d21 may though vary between the differentlayers 1121, see e.g. FIG. 4 b . The three layers 1121 a, 1121 b and1121 c may e.g. depict alternating Al₂O₃ layers (by way of example 1121b) and Ta₂O₅ layers (by way of example 1121 a,1121 c). The (further sub)layer coating layer thickness (not indicated with a reference) of the(further sub) layers 1141 of the further multilayer 1140 may especiallybe in the ranges as described in relation to the layer coating layerthickness d21 of the layers 1121 of the multilayer 1120.

FIG. 2 a further schematically depicts that the primer layer 105comprises an oxide-containing layer 101 and a primary sol-gel layer 110.The oxide-containing layer 101 is arranged at a surface 67 of the core102. In the embodiment, the oxide-containing layer 101 and the primarysol-gel layer 110 are continuous and conformal. Yet, in furtherembodiments, this may not be the case, and e.g. the main ALD coatinglayer 120 may contact the oxide-containing layer 101 at some locationsand may even contact the surface 67 of the core at some further location(while contacting the primary sol-gel layer 110 at other locations.

FIG. 2 a further indicates with references 17, 27, 37, 47, 57 thesurfaces of respective layers, and with reference 67 the surface of thecore 102. As indicated above, the layer thicknesses described herein areespecially average layer thicknesses. Especially at least 50%, even moreespecially at least 80%, of the area of the respective layers have suchindicated layer thickness. Hence, referring to the thickness d2 betweensurface 47 and surface 37, below at least 50% of surface 37, a layerthickness in the range of e.g. 5-250 nm may be found, with the otherless than at least 50% of the surface area 37 e.g. smaller or largerthicknesses may be found, but in average d2 of the main ALD coating(multi-)layer 120 is in the indicated range of 5-250 nm. Likewise, thismay apply to the other herein indicated thicknesses. For instance,referring to the thickness d3 between surface 37 and surface 27, thisthickness may over at least 50% of the area of 27 be in the range of50-700 nm, with the other less than at least 50% of the surface area 27e.g. smaller or larger thicknesses may be found, but in average d1 ofthe first layer main sol-gel layer 130 is in the indicated range of50-700 nm, such as especially 100-500 nm.

FIG. 2 b schematically depicts an embodiment wherein the luminescentcore 102 includes a luminescent nanoparticle, here by way of example aquantum dot 160. The quantum dot in this example comprises a quantum rodwith a (semiconductor) core material 161, such as ZnSe, and a shell 162,such as ZnS. Of course, other luminescent nanoparticles may also beused. Such luminescent quantum dot 160 can also be provided with thehybrid coating.

FIGS. 1-2 schematically depict luminescent particles 100 having a singlenucleus. However, optionally also aggregates encapsulated with thehybrid coating may be formed. This may especially apply for quantum dotsas luminescent particles defining the luminescent core 102.

The figures especially depict embodiments of the coating architecture onphosphor particles or luminescent cores 102 (after applying therespective (ALD and sol-gel) coating processes). The phosphor particles102 may be covered by an oxide layer 101 formed by a washing and bakingprocess. The primary sol-gel coating 110 comprises in embodimentssilicon oxide (SiO₂) provided by a (primary) sol-gel coating process.The first SiO₂ layer 110 especially acts as nucleation or seed layer forthe main ALD coating layer 120, provided by a main atomic layerdeposition process. Therefore, (the primary layer 105 as well as) theprimary sol-gel coating layer 110 does not need to form a conformal orfully closed coating around each core 102. The primary sol-gel coatinglayer 110, e.g. the primary SiO₂ layer 110 can also be seen as a surfacetreatment to provide OH-groups on the phosphor particles 102. SuchOH-groups may assist the ALD precursors to bond on the surface andconsequently initiate film growth.

The main ALD coating layer 120 especially comprises a multilayer 1120also called “nanolaminate” 1120 of metal oxides (sub-)layers 1121. Ananolaminate 1120 may form an extremely dense and nearly pinhole freeconformal coating on phosphor particles that is almost impermeable togases like water vapor and oxygen. The nanolaminate protection layer1120 may in embodiments have a thickness d2 of 20-50 nm consisting ofmore than two sub-layers of Al₂O₃, TiO₂, ZrO₂, HfO₂, SnO₂, ZnO or Ta₂O₅.Each layer 1121 may have a thickness d21 in the range of 1 nm-15 nm. Theouter layer 1121, i.e. the layer (1121 a in FIGS. 1 and 2 a) contactingthe main sol gel coating layer 130 is in embodiments a chemical stablelayer such as HfO₂, ZrO₂ or Ta₂O₅ that does not corrode when exposed towater or other solvents such as cyclohexanone.

The main sol-gel coating layer 130 may also comprise silicon oxide(SiO₂) provided by the (main) sol-gel coating process, analog to theprimary sol-gel coating layer 110. The main sol-gel coating 130 mayespecially function as mechanical protection to prevent damage of theunderlying barrier coating 120. In an LED fabrication process phosphorparticles undergo various process steps, such as mixing, sieving,pressing, and molding. These process steps may induce mechanical stressin the coating. As a results the coating might be damaged. The mainsol-gel coating layer 130 provides a high robustness againstpost-processing and fabrication steps. In embodiments a high reliabilitycan be guaranteed by applying the main sol-gel coating layer 130 layeron the luminescent particles 100.

In embodiments of the invention, a further ALD coating layer 140 isadded to the layer architecture, as depicted in FIG. 2 a . The furtherALD coating layer 140 in the embodiment comprises a nanolaminate 1140.The layer 140 or multilayer 1140 may comprise metal oxides such asAl₂O₃, TiO₂, ZrO₂, HfO₂, SnO₂, ZnO or Ta₂O₅. The total thickness d4 ofthe layer 140 is especially in the range of 10-50 nm. The further ALDcoating layer 140 may further stabilize the overall coating structure byfilling pores and pin-holes in the main sol-gel coating layer 130. Inaddition, the further ALD coating layer 140 can suppress the surfacereactivity of the main sol-gel layer 130. This surface reactivity may bein embodiments of LED manufacturing processes be advantageous formaintaining the rheology or other properties of certain siliconephosphor slurries.

FIG. 3 schematically depicts a lighting device 20 comprising a lightsource 10 configured to generate light source radiation 11, especiallyone or more of blue and UV, as well as a wavelength converter 30comprising the luminescent material 1 with particles 100 as definedherein. The wavelength converter 30 may e.g. comprise a matrix, such asa silicone or organic polymer matrix as described above, with the coatedparticles 100 embedded therein. The wavelength converter 30 isconfigured to (wavelength) convert at least part of the light sourceradiation 11 into wavelength converter light 31. Optionally also lightsource radiation 11 may pass the wavelength converter 30 (without beingconverted). The wavelength converter light 31 at least includesluminescence from the herein described coated particles 100. However,the wavelength converter 30 may optionally include also one or moreother luminescent materials. The wavelength converter 30, or moreespecially the luminescent material 1, may be arranged at a non-zerodistance d30, such as at a distance of 0.1-100 mm. However, optionallythe distance d30 may be zero, such as e.g. when the luminescent materialis embedded in a dome on a LED die. The distance d30 is the shortestdistance between a light emitting surface of the light source 10, suchas a LED die, and the wavelength converter 30, more especially theluminescent material 1.

The light source 10 may be an LED, such that lighting device 20 is aphosphor-converter LED (“pcLED”). For example, light source 10 may be aIII-Nitride LED that emits ultraviolet, blue, green, or red light. LEDsformed from any other suitable material system and that emit any othersuitable wavelength of light may also be used. Other suitable materialsystems may include, for example, III-Phosphide materials, III-Arsenidematerials, and II-VI materials.

FIG. 4 a shows a SEM image of luminescent material 1 comprising somecoated luminescent particles 100. In FIG. 4 b a TEM image of a coatedluminescent particle 100 is given, clearly showing or core 102 with anoxide-containing layer 101, a primary (SiO₂) sol-gel coating layer 110,a main ALD coating layer 120, comprising a multilayer 1120 consisting oftwo Al₂O₃ layers 1121 b, and two Ta₂O₅ layers 1121 a, and a (SiO₂) mainsol-gel coating 130.

FIGS. 5 a-5 b show some experimental results. In the figures, coatedluminescent particles 100 of the invention (here comprisingSrLiAl₃N₄:Eu) are compared to corresponding prior art luminescentparticles. The prior art luminescent particles also comprise an ALDcoating layer and a sol-gel coating layer. However, the sol-gel coatinglayer is configured directly at the surface of the luminescent core 102,and the ALD coating layer is configured onto the sol-gel coating.

In FIG. 5 a . the (normalized) light output (Y-axis) over time,especially hours (X-axis) of the respective luminescent particles insilicone is given. During the experiment, the particles were kept at130° C. and 100% relative humidity. The circular markers indicate theluminescent particle 100 of the invention; the square markers indicatethe prior art luminescent particle.

In FIG. 5 b . the failure probability of white LEDs with the respectiveluminescent particles is given after maintaining the respective LEDsover 500 hours at 85° C. and 85% relative humidity. The square markersindicate the luminescent particle 100 of the invention; the circularmarkers indicate the prior art luminescent particle. Note that theprobability is given in percentages at the Y-axis in a logarithmicalscale. The color point shift in Δu′v′ (sometimes also indicated as“(du′v′)” or “duv”) is given at the X-axis. The (LEDs comprising the)luminescent particles 100 of the invention clearly show less color shift(Δu′v′ is calculated as the Euclidian distance between a pair ofchromaticity coordinates in the (u′, v′) CIE 1976 color space).

Hence, this invention concerns methods to improve the barrier propertiesof phosphor particle coatings. While the invention is generallyapplicable to various phosphor particles, it is particularly suitablefor nitride based narrow-band, red-emitting phosphors like nitridealuminates or oxo nitride aluminates due to their high sensitivityagainst moisture.

FIGS. 6A-6B show, respectively, cross-sectional and top views of anarray 600 of pcLEDs 610, which pcLEDs 610 may be structured as lightingdevice 20, as shown in FIG. 3 , that include a wavelength converter 30comprising the coated luminescent particles 100 as defined hereinincluded in phosphor pixels 606 with semiconductor diode 612 disposed ona substrate 602. Such an array may include any suitable number of pcLEDsarranged in any suitable manner. In the illustrated example the array isdepicted as formed monolithically on a shared substrate, butalternatively an array of pcLEDs may be formed from separate individualpcLEDs. Substrate 602 may optionally comprise CMOS circuitry for drivingthe LED and may be formed from any suitable materials.

Although FIGS. 6A-6B, show a three-by-three array of nine pcLEDs, sucharrays may include for example tens, hundreds, or thousands of LEDs.Individual LEDs (pixels) may have widths (e.g., side lengths) in theplane of the array, for example, less than or equal to 1 millimeter(mm), less than or equal to 500 microns, less than or equal to 100microns, or less than or equal to 50 microns. LEDs in such an array maybe spaced apart from each other by streets or lanes having a width inthe plane of the array of, for example, hundreds of microns, less thanor equal to 100 microns, less than or equal to 50 microns, less than orequal to 10 microns, or less than or equal to 5 microns. Although theillustrated examples show rectangular pixels arranged in a symmetricmatrix, the pixels and the array may have any suitable shape orarrangement.

LEDs having dimensions in the plane of the array (e.g., side lengths) ofless than or equal to about 50 microns are typically referred to asmicroLEDs, and an array of such microLEDs may be referred to as amicroLED array.

An array of LEDs, or portions of such an array, may be formed as asegmented monolithic structure in which individual LED pixels areelectrically isolated from each other by trenches and/or insulatingmaterial, but the electrically isolated segments remain physicallyconnected to each other by portions of the semiconductor structure.

The individual LEDs in an LED array may be individually addressable, maybe addressable as part of a group or subset of the pixels in the array,or may not be addressable. Thus, light emitting pixel arrays are usefulfor any application requiring or benefiting from fine-grained intensity,spatial, and temporal control of light distribution. These applicationsmay include, but are not limited to, precise special patterning ofemitted light from pixel blocks or individual pixels. Depending on theapplication, emitted light may be spectrally distinct, adaptive overtime, and/or environmentally responsive. Such light emitting pixelarrays may provide pre-programmed light distribution in variousintensity, spatial, or temporal patterns. The emitted light may be basedat least in part on received sensor data and may be used for opticalwireless communications. Associated electronics and optics may bedistinct at a pixel, pixel block, or device level.

As shown in FIGS. 7A-7B, a pcLED array 600 may be mounted on anelectronics board 700 comprising a power and control module 702, asensor module 704, and an LED attach region 706. Power and controlmodule 702 may receive power and control signals from external sourcesand signals from sensor module 704, based on which power and controlmodule 702 controls operation of the LEDs. Sensor module 704 may receivesignals from any suitable sensors, for example from temperature or lightsensors. Alternatively, pcLED array 600 may be mounted on a separateboard (not shown) from the power and control module and the sensormodule.

Individual pcLEDs may optionally incorporate or be arranged incombination with a lens or other optical element located adjacent to ordisposed on the phosphor layer. Such an optical element, not shown inthe figures, may be referred to as a “primary optical element”. Inaddition, as shown in FIGS. 8A-8B a pcLED array 600 (for example,mounted on an electronics board 700) may be arranged in combination withsecondary optical elements such as waveguides, lenses, or both for usein an intended application. In FIG. 8A, light emitted by pcLEDs 610 iscollected by waveguides 802 and directed to projection lens 804.Projection lens 804 may be a Fresnel lens, for example. This arrangementmay be suitable for use, for example, in automobile headlights. In FIG.8B, light emitted by pcLEDs 610 is collected directly by projection lens804 without use of intervening waveguides. This arrangement may beparticularly suitable when pcLEDs can be spaced sufficiently close toeach other and may also be used in automobile headlights as well as incamera flash applications. A microLED display application may usesimilar optical arrangements to those depicted in FIGS. 8A-8B, forexample. Generally, any suitable arrangement of optical elements may beused in combination with the LED arrays described herein, depending onthe desired application.

An array of independently operable LEDs may be used in combination witha lens, lens system, or other optical system (e.g., as described above)to provide illumination that is adaptable for a particular purpose. Forexample, in operation such an adaptive lighting system may provideillumination that varies by color and/or intensity across an illuminatedscene or object and/or is aimed in a desired direction. A controller canbe configured to receive data indicating locations and colorcharacteristics of objects or persons in a scene and based on thatinformation control LEDs in an LED array to provide illumination adaptedto the scene. Such data can be provided for example by an image sensor,or optical (e.g. laser scanning) or non-optical (e.g. millimeter radar)sensors. Such adaptive illumination is increasingly important forautomotive, mobile device camera, VR, and AR applications.

FIG. 9 schematically illustrates an example camera flash system 900comprising an LED array and lens system 902, which may be similar oridentical to the systems described above. Flash system 900 alsocomprises an LED driver 906 that is controlled by a controller 904, suchas a microprocessor. Controller 904 may also be coupled to a camera 907and to sensors 908, and operate in accordance with instructions andprofiles stored in memory 910. Camera 907 and adaptive illuminationsystem 902 may be controlled by controller 904 to match their fields ofview.

Sensors 908 may include, for example, positional sensors (e.g., agyroscope and/or accelerometer) and/or other sensors that may be used todetermine the position, speed, and orientation of system 900. Thesignals from the sensors 908 may be supplied to the controller 904 to beused to determine the appropriate course of action of the controller 904(e.g., which LEDs are currently illuminating a target and which LEDswill be illuminating the target a predetermined amount of time later).

In operation, illumination from some or all pixels of the LED array in902 may be adjusted—deactivated, operated at full intensity, or operatedat an intermediate intensity. Beam focus or steering of light emitted bythe LED array in 902 can be performed electronically by activating oneor more subsets of the pixels, to permit dynamic adjustment of the beamshape without moving optics or changing the focus of the lens in thelighting apparatus.

FIG. 10 schematically illustrates an example display (e.g., AR/VR/MR)system 1000 that includes an adaptive light emitting array 1010, display1020, a light emitting array controller 1030, sensor system 1040, andsystem controller 1050. Control input is provided to the sensor system1040, while power and user data input is provided to the systemcontroller 1050. In some embodiments modules included in system 1000 canbe compactly arranged in a single structure, or one or more elements canbe separately mounted and connected via wireless or wired communication.For example, the light emitting array 1010, display 1020, and sensorsystem 1040 can be mounted on a headset or glasses, with the lightemitting controller and/or system controller 1050 separately mounted.

The light emitting array 1010 may include one or more adaptive lightemitting arrays, as described above, for example, that can be used toproject light in graphical or object patterns that can support AR/VR/MRsystems. In some embodiments, arrays of microLEDs can be used.

System 1000 can incorporate a wide range of optics in adaptive lightemitting array 1010 and/or display 1020, for example to couple lightemitted by adaptive light emitting array 1010 into display 1020.

Sensor system 1040 can include, for example, external sensors such ascameras, depth sensors, or audio sensors that monitor the environment,and internal sensors such as accelerometers or two or three axisgyroscopes that monitor an AR/VR/MR headset position. Other sensors caninclude but are not limited to air pressure, stress sensors, temperaturesensors, or any other suitable sensors needed for local or remoteenvironmental monitoring. In some embodiments, control input can includedetected touch or taps, gestural input, or control based on headset ordisplay position.

In response to data from sensor system 1040, system controller 1050 cansend images or instructions to the light emitting array controller 1030.Changes or modification to the images or instructions can also be madeby user data input, or automated data input as needed. User data inputcan include but is not limited to that provided by audio instructions,haptic feedback, eye or pupil positioning, or connected keyboard, mouse,or game controller.

In an embodiment, the invention provides a wet chemical washing(including drying) process of the powder phosphor (the luminescentcore(s)) to form an oxide outer particle layer. Further a primary (SiO₂)sol-gel layer may be deposited by a (primary) sol-gel process to providethe primary sol-gel layer with a thickness in the range of 0.5-5 nm.Next, a multilayer may be deposited by ALD with a total ALD coatinglayer thickness d2 in embodiments of 20-50 nm and a (sub)layer thicknessd21 of the layers 1121 of the multilayer 1120 in the range of 1-20 nm.The multilayer 1120 is especially comprised of two or more metal oxidessuch as Al₂O₃, TiO₂, ZrO₂, HfO₂, SnO₂, ZnO, Ta₂O₅. Next, a third layer,especially a main sol-gel coating layer 130, e.g. of SiO₂ may bedeposited by a (main) sol-gel process with a thickness in the range of100-500 nm. In yet further embodiments, a fourth layer 140 may bedeposited by a further ALD process. The further ALD coating layer 140may in embodiments have a total thickness d4 of 5-50 nm and especiallymay comprise a multilayer with sub-layer thickness in the range of 1-20nm. The multilayer is in embodiments comprised of one or more metaloxides, such as Al₂O₃, TiO₂, ZrO₂, HfO₂, SnO₂, ZnO, Ta₂O₅.

EXPERIMENTAL

The effect of the new coating architecture of the invention was testedby forming a luminescent particle with a hybrid coating as disclosedherein:

Polyol Washing Process: 10.3 g of a Raw Phosphor Powder Sample ofComposition

Sr_(0.995)Li₂Al_(1.995)Si_(0.005)O_(1.995)N_(2.005):Eu_(0.005) was mixed30.0 g ethanol and 30.0 g triethylene glycol with the suspension showinga total water content in the 0.05-0.1% range in an ultrasonic bathfollowed by a 16 hr treatment at 80° C. in a closed pressure vessel.After cooling down to room temperature, the phosphor powder was washedwith ethanol and dried at 100° C. under ambient atmosphere.

Mixed Solvent Acetic Acid Washing Process:

200-250 g of SrLiAl₃N₄:Eu_(0.007) were stirred in 837 g isopropanol. 560g of 18.5 wt % acetic acid were slowly added under stirring. Thesuspension was further stirred until a total time of 40 min (includingacid addition) passed. After 30 min sedimentation the supernatant waslargely removed by decantation followed by filtration and rinsing withacetic acid/isopropanol mixture and isopropanol. The washed phosphor isfinally dried at 50° C. in vacuum overnight.

Thin Amorphous Silica Layer (<5 nm)

In this experiment a primary sol-gel coating layer was provided. 200 gphosphor powder (typically after washing) were stirred in 960 g ethanol.To this suspension 3.5 g tetraethyl orthosilicate were added and stirredfor 10 min under sonication. 90 g 25 wt % aqueous ammonia solution wereadded and stirring under sonication is continued for another 20 min.Fine particles including nanosized silica particles formed as by-productwere removed by threefold sedimentation in ethanol and decantation. Thecoated powder was dried at 50° C. in vacuum overnight. After dry-sieving(mesh size 100 μm) the coating was cured by heating the powder to 300°C. for 10 hr. under vacuum.

ALD Nanolaminate (˜25 nm)

Next, a main ALD coating layer comprising an ALD nanolaminate wasapplied on primer layer comprising phosphor particles (comprisingSrLiAl₃N₄:Eu) in a Picosun Oy ALD R200 reactor. Precursor materials weretrimethylaluminum and H₂O to form an Al₂O₃ film and(tert-Butylimido)tris(ethylmethylamino) tantalum (V) and H₂O to form aTa₂O₅ film. The deposition temperature was set to 250° C. The purge timeof nitrogen gas in between precursor pulses was 60 seconds. Thenanolaminate consists of 2×Al₂O₃/Ta₂O₅ sublayers with a total thicknessof around 25 nm.

Thick Amorphous Silica Layer (˜170 nm)

In this experiment a main sol-gel coating layer was provided on theluminescent particle. 85 g powder (typically after ALD coating) werestirred in 672 g ethanol for 15 min under sonication. To thissuspension 1) 116 g 25 wt % aqueous ammonia solution were added fast(<30 s) and 2) a solution of 68 g tetraethyl orthosilicate in 408 gethanol is added drop-wise (˜45 min). After the addition of alkoxideprecursor was finished, the suspension was stirred for another 30 minwithout sonication.

Fine particles including sub-micron sized silica particles formed asby-product were removed by threefold sedimentation in ethanol anddecantation. The coated powder was dried at 50° C. in vacuum overnight.After dry-sieving (mesh size 63 μm) the coating was cured by heating thepowder to 300° C. for 10 hr. under vacuum.

A SEM image of some of the particles is given in FIG. 4 a . A TEM imageof the particles is given in FIG. 4 b.

Comparison Test

The prepared particles in silicone were subjected to a stress test andcompared with a control particles i.e. particles comprising a prior artcoating architecture. In the prior art coating architecture theluminescent particle is initially coated with a relatively thick sol-gelcoating and successively with a thin ALD coating. In the stress test,the light output was measured over time while keeping the particles at atemperature of 130° C. and 100% relative humidity.

The prepared particles were further applied in a white LED and stressedover 500 hours at 85° C. and 85% relative humidity. The failureprobability of the white LEDs with the luminescent particles accordingto the invention was compared to the failure probability of white LEDscomprising the prior art coating architecture subjected to the samestress test (the control LED).

The results are depicted in FIGS. 5 a-5 b showing a significantlyimproved reduction in light output after 60 hours stress test, i.e. lessthan 5% compared to a reduction of more than 50% for the controlparticles. Also the color shift (Δu′v′) is substantially minimizedcompared to the control LED.

The term “plurality” refers to two or more.

The terms “substantially” or “essentially” herein, and similar terms,will be understood by the person skilled in the art. The terms“substantially” or “essentially” may also include embodiments with“entirely”, “completely”, “all”, etc. Hence, in embodiments theadjective substantially or essentially may also be removed. Whereapplicable, the term “substantially” or the term “essentially” may alsorelate to 90% or higher, such as 95% or higher, especially 99% orhigher, even more especially 99.5% or higher, including 100%.

The term “comprise” also includes embodiments wherein the term“comprises” means “consists of”.

The term “and/or” especially relates to one or more of the itemsmentioned before and after “and/or”. For instance, a phrase “item 1and/or item 2” and similar phrases may relate to one or more of item 1and item 2. The term “comprising” may in an embodiment refer to“consisting of” but may in another embodiment also refer to “containingat least the defined species and optionally one or more other species”.

Furthermore, the terms first, second, third and the like in thedescription and in the claims, are used for distinguishing betweensimilar elements and not necessarily for describing a sequential orchronological order. It is to be understood that the terms so used areinterchangeable under appropriate circumstances and that the embodimentsof the invention described herein are capable of operation in othersequences than described or illustrated herein.

The devices, apparatus, or systems may herein amongst others bedescribed during operation. As will be clear to the person skilled inthe art, the invention is not limited to methods of operation, ordevices, apparatus, or systems in operation.

It should be noted that the above-mentioned embodiments illustraterather than limit the invention, and that those skilled in the art willbe able to design many alternative embodiments without departing fromthe scope of the appended claims.

In the claims, any reference signs placed between parentheses shall notbe construed as limiting the claim.

Use of the verb “to comprise” and its conjugations does not exclude thepresence of elements or steps other than those stated in a claim. Unlessthe context clearly requires otherwise, throughout the description andthe claims, the words “comprise”, “comprising”, and the like are to beconstrued in an inclusive sense as opposed to an exclusive or exhaustivesense; that is to say, in the sense of “including, but not limited to”.

The article “a” or “an” preceding an element does not exclude thepresence of a plurality of such elements.

The invention may be implemented by means of hardware comprising severaldistinct elements, and by means of a suitably programmed computer. In adevice claim, or an apparatus claim, or a system claim, enumeratingseveral means, several of these means may be embodied by one and thesame item of hardware. The mere fact that certain measures are recitedin mutually different dependent claims does not indicate that acombination of these measures cannot be used to advantage.

The invention also provides a control system that may control thedevice, apparatus, or system, or that may execute the herein describedmethod or process. Yet further, the invention also provides a computerprogram product, when running on a computer which is functionallycoupled to or comprised by the device, apparatus, or system, controlsone or more controllable elements of such device, apparatus, or system.

The invention further applies to a device, apparatus, or systemcomprising one or more of the characterizing features described in thedescription and/or shown in the attached drawings. The invention furtherpertains to a method or process comprising one or more of thecharacterizing features described in the description and/or shown in theattached drawings.

The various aspects discussed in this patent can be combined in order toprovide additional advantages. Further, the person skilled in the artwill understand that embodiments can be combined, and that also morethan two embodiments can be combined. Furthermore, some of the featurescan form the basis for one or more divisional applications.

1. A method for providing a luminescent particle with a hybrid coating,the method comprising: providing a particulate luminescent materialhaving a surface; forming a primer layer directly on at least a portionof the surface; performing a first atomic layer deposition process onthe particulate luminescent material having the primer layer to deposita first ALD layer, the first atomic layer deposition process using ametal oxide first precursor selected from a group of metal oxidescomprising Al, Zn, Hf, Ta, Zr, Ti, Sn, Nb, Y, Ga, and V; performing asecond atomic layer deposition process to deposit a second ALD layeronto the first ALD layer, the second atomic layer deposition processusing a metal oxide second precursor different from the first precursorand selected from a group of metal oxides comprising Al, Zn, Hf, Ta, Zr,Ti, Sn, Nb, Y, Ga, and V; and performing a main sol-gel coating processto form a main sol-gel coating layer directly onto the second ALD layer,the main sol-gel coating layer having a chemical composition differentfrom the first ALD layer and the second ALD layer.
 2. The methodaccording to claim 1, wherein forming the primer layer directly onto atleast a portion of the surface of the particulate luminescent materialcomprises performing a primer sol-gel coating process on the particulateluminescent material, the primer sol-gel coating process using a metalalkoxide precursor.
 3. The method according to claim 2, wherein theprimer sol-gel coating process comprises: suspending the particulateluminescent material in an alcohol-aqueous ammonia solution mixture;adding the metal alkoxide precursor to the mixture; stirring the mixturecontaining the metal alkoxide until the primer layer is formed; washingthe particulate luminescent material having the primer layer withalcohol; and drying the particulate luminescent material having theprimer layer.
 4. (canceled)
 5. The method according to claim 1, furthercomprising washing the particulate luminescent material in a washingsolvent before forming the primer layer, the washing solvent having apH<7. 6-7. (canceled)
 8. The method according to claim 5, wherein thewashing solvent comprises a weak acid and equal to or less than 50%wt/wt water, and washing the particulate luminescent material furthercomprises: successively subjecting the particulate luminescent materialto a drying treatment.
 9. (canceled)
 10. The method according to claim1, wherein: the primer layer has a primer layer thickness (d1) in therange of 0.1-5 nm, the first ALD layer and second ALD layer form a mainALD coating layer, and the main ALD coating layer has a main ALD coatinglayer thickness (d2) in the range of 5-250 nm; and the main sol-gelcoating layer has a main sol-gel coating layer thickness (d3) in therange of 50-700 nm.
 11. (canceled)
 12. The method according to claim 1,wherein the main sol-gel coating process comprises: providing a mixtureof an alcohol, ammonia, water, the particulate luminescent materialhaving the primer layer and the first ALD layer and the second ALDlayer, and a metal alkoxide precursor while agitating the mixture, andallowing a main sol-gel coating layer to be formed directly onto thesecond ALD layer, the metal alkoxide precursor is titanium alkoxide,silicon alkoxide, and or aluminum alkoxide; and retrieving theparticulate luminescent material having the primer layer, the first ALDlayer and the second ALD layer and the main sol-gel coating layer fromthe mixture and subjecting the retrieved particulate luminescentmaterial having the primer layer, the first ALD layer and the second ALDlayer and the main sol-gel coating layer to a heat treatment. 13.(canceled)
 14. The method according to claim 1, comprising: successivelyproviding n additional ALD layers, wherein 2≤n≤10, between the first ALDlayer and the second ALD layer, each additional ALD layer has anadditional ALD layer coating layer thickness (d21) in the range of 1-20nm, one or more additional ALD layers comprise one or more metal oxidesselected from a group of HfO₂, ZrO₂, TiO₂, and Ta₂O₅, one or moreadditional ALD layers comprise Al₂O₃, and the second ALD consist of oneor more metal oxides selected from the group of HfO₂, ZrO₂, TiO₂, andTa₂O₅.
 15. The method according to claim 1, further comprising:providing a further ALD coating layer onto the main sol-gel coating byapplication of a further atomic layer deposition process, in the furtheratomic layer deposition process a further metal oxide precursor isselected from a group of metal oxides comprising Al, Zn, Hf, Ta, Zr, Ti,Sn, Nb, Y, Ga, and V, the further ALD coating layer has a further ALDcoating layer thickness (d4) in the range of 10-50 nm, and the furtherALD coating layer comprises two or more layers having different chemicalcompositions, one or more of the layers comprise metal oxides selectedfrom a group of Al₂O₃, TiO₂, ZrO₂, HfO₂, SnO₂, ZnO and Ta₂O₅, and thetwo or more layers have a chemical composition differing from thechemical composition of the main sol-gel coating layer.
 16. (canceled)17. The method according to claim 1, wherein the surface of theparticulate luminescent material comprises an alkaline earth element,aluminum, and oxide, wherein the alkaline earth element comprisesstrontium.
 18. (canceled)
 19. (canceled)
 20. The method according toclaim 1, wherein the particulate luminescent material is selected from agroup consisting of (i) the SrLiAl₃N₄:Eu²⁺ class, and (ii) theSrLi₂Al_(1.995)Si_(0.005)O_(1.995)N_(2.005):Eu²⁺ class. 21-22.(canceled)
 23. A luminescent material comprising: a particulateluminescent material having a surface; a primer layer disposed on and incontact with the surface of the particulate luminescent material, theprimer layer comprising a primer layer metal oxide and having athickness in the range of 0.1-5 nm; a first ALD layer disposed on and incontact with the primer layer and any portion of the surface of theparticulate luminescent material not covered with the primer layer, thefirst ALD layer comprising a first oxide of one or more of Al, Zn, Hf,Ta, Zr, Ti, Sn, Nb, Y, Ga, and V, and different from the primer layermetal oxide; a second ALD layer disposed on and in contact with thefirst ALD layer, the second ALD layer comprising a second oxide of oneor more of Al, Zn, Hf, Ta, Zr, Ti, Sn, Nb, Y, Ga, and V, and differentfrom the first oxide, the first ALD layer and second ALD layer forming amain ALD layer having a thickness in the range of 5-250 nm; and a mainsol-gel coating layer disposed on the second ALD layer, wherein the mainsol-gel coating has a main sol-gel coating layer thickness (d3) in therange of 50-700 nm, wherein the main sol-gel coating layer has achemical composition differing from the first ALD layer and the secondALD layer.
 24. The luminescent material according to claim 23, whereinat least a portion of the surface of the particulate luminescentmaterial comprises an oxide.
 25. The luminescent material according toclaim 24, wherein at least a portion of the surface of the particulateluminescent material comprises an alkaline earth element and aluminum,wherein the alkaline earth element comprises strontium. 26-27.(canceled)
 28. The luminescent material according to claim 23, whereinthe particulate luminescent material is selected from a group consistingof (i) the SrLiAl₃N₄:Eu²⁺ class, and (ii) theSrLi₂Al_(1.995)Si_(0.000)O_(1.995)N_(2.005):Eu²⁺ class.
 29. Theluminescent material according to claim 23, further comprising a furtherALD coating layer arranged onto the main sol-gel coating layer, thefurther ALD coating layer having a further ALD coating layer thickness(d4) in the range of 10-50 nm, the further ALD coating layer comprises afurther multilayer with two or more layers having different chemicalcompositions, one or more of the layers comprising metal oxides selectedfrom a group of Al₂O₃, TiO₂, ZrO₂, HfO₂, SnO₂, ZnO and Ta₂O₅, and thetwo or more layers having a chemical composition differing from thechemical composition of the main sol-gel coating layer. 30-31.(canceled)
 32. The luminescent material according to claim 23, furthercomprising n additional ALD layers, wherein 2≤n≤10, between the firstALD layer and the second ALD layer, each additional ALD layer having anadditional ALD layer coating layer thickness (d21) in the range of 1-20nm, and one or more of the additional ALD layers comprise one or moremetal oxides selected from a group of HfO₂, ZrO₂, TiO₂, and Ta₂O₅.33-35. (canceled)
 36. A display system comprising: a light emittingdiode array, the light emitting diode array including a plurality ofphosphor converted light emitting diodes, each phosphor converted lightemitting diode including a wavelength converter comprising theluminescent material of claim 23; a display; and a lens or lens systemspaced apart from the light emitting diode array and arranged to couplelight from the light emitting diode array into the display.
 37. A mobiledevice comprising: a camera; and a flash illumination system comprising:a light emitting diode array including a plurality of light emittingdiodes, each light emitting diode including a wavelength convertercomprising the luminescent material of claim 23.